Part IV

flower cluster (head) of a sunflower, with the ovary, causing the outer parts of the flower to be borne above the ovary, is an advanced feature; the union of the petals into a corolla tube and that of the anthers into an anther tube also are advanced characters. The occurrence of flowers of two distinct types in the same head is likewise a highly specialized condition. In this latter respect, however, the sunflower is not typical of all composites. In the dandelion, as in a number of related genera, the head contains flowers of only one type which are similar in corolla form to the ray flowers of the sunflower. Among the comparatively few members of the family that supply food for man are the lettuce, endive, chicory, salsify, artichoke, and Jerusalem artichoke. The sunflower is used as food 478 GENERAL BOTANY for livestock. Drugs are obtained from some composites, including camomile, calendula, arnica, tansy, and wormwood. Among orna- mental plants of the family are the daisies, sunflower, dahlia, asters, and chrysanthemums. Some of the commonest wild plants and weeds, among them being conspicuous members of the autumnal flora, are asters, golden-rods, ragweeds, and thistles, the sagebrush, dandelion, beggar-ticks, yarrow, cocklebur, and burdock. MONOCOTYLEDONS 365. Cat-tail Family. The monocotyledons seem to have arisen from some very primitive dicotyledon or dicotyledons. Within the class of monocotyledons, the course of evolution has substantially paralleled that which has marked the history of dicotyledons. In consequence, while preserving the characteristics that distinguish them from dicotyledons ( 336), monocotyledons show very much the same steps in the evolution of floral structures that have been described for dicotyledons. The small cat-tail family is one of the most primitive among living monocotyledons, and may be thought of as holding much the same position in this class that the willow family occupies FIG. 384. Cat-tail, An "among dicotyledons. aerial branch, a single The characteristics of the family are leaf, and a flower clus- essenti ally those of the common cat-tail er (Fig. 384), growing abundantly in wet, marshy places. This plant has a branching horizontal stem that lives in the mud from year to year, and each spring sends up aerial branches. Each such branch bears at its base long, sheathing leaves. At the upper end of an aerial branch is a long cylindrical spike of flowers. The central axis of a close cylin- drical spike of this nature is a spadix. The flowers on the spadix are partly covered while young by long, thin, sheathing bracts (spathes); one spathe arises from the base of the spike, and FLORAL TYPES 479 others may appear higher up, interrupting the cylindrical mass of flowers. The flowers in the upper part of the spike are staminate, those in the lower part pistillate. Each staminate flower consists of two or three stamens borne on a short pedicel from whose lower part A arise a number of hair-like outgrowths. pistillate flower has a single pistil consisting of one carpel borne, like the stamens, upon a short, hairy pedicel. The ovary contains one ovule. After pol- lination, which is brought about by winds, the staminate flowers wither and disappear, leaving the upper part of the spadix bare. Each ovary may de- velop into an achene; the pedicel with its many hairs remains attached to the fruit when the latter is shed, and Palet the hairs assist in the distri- bution of the fruit by winds. 366. Grass Family. Here belong about 4,500 species which, like the cat-tails, have small, simple flowers and oneseeded fruits. In various re- \tigma Stamm. spects, however, grasses show a considerably greater degree of specialization than do cattails, and they are very much more widely spread, different species being adapted to very different habitats. Like most FIG. 385. ^Lodicuie Wheat. A, spikelet. B, single flower. monocotyledons they are herbaceous, although the tall, almost tree-like bamboos have more or less woody stems. The stems of grasses are jointed, the internodes being commonly hollow, and the leaves are alternately arranged in two vertical rows. Economically the most important grasses are the cereal grains, which include wheat, oats, barley, rye, corn, rice, and millet. The flower of wheat (Fig. 385) may be taken as typical. The compound flower cluster, commonly called a head or spike, is made up of many small spikelets. Beginning at the base of a spikelet, and alternating on opposite sides of its central axis, are 480 GENERAL BOTANY two rather large bracts (empty glumes), and successively above these a few progressively smaller glumes (lemmas), each with a A flower in its axis. lemma has its concave face toward the axis of the spikelet, and the lower lemmas may bear long, stiff bristles (awns). Partly enclosed by each lemma is a thin bract (pakl) which envelops the flower proper. The flower includes a pistil with a short ovary, and two short styles each terminating in a long, feathery stigma; three sta- mens with long anthers and thread-like filaments and ; two small scales (lodicules) which may correspond to sepals. The ovary with its single ovule develops into a grain. In the corn and a few re- lated grasses, stamens and pistils are borne in separate flowers; the staminate flower cluster of the corn is the tassel; the pistillate flower cluster is the ear. In addition to cereals, the grasses of most practical in- terest are the sugar cane, A Fio. 386. sedge. sorghum, and broom corn; the bamboos, which in their native countries are used for a great variety of purposes; and many species which, like red top and timothy, are used for forage. The value of wild grasses for pasturage results in large part from their habit of growing together in great numbers, so that a considerable area may be covered by one or a few species. Their power of rapid multiplication by means of seeds as well as by the growth and branching of their underground stems makes some of the grasses, like so many of the composites, troublesome weeds. Some familiar weeds of this family are the wild oat, quack grass, and chess. FLORAL TYPES 481 367. Sedge Family. Very similar to grasses in general appear- ance and in many characteristics are the sedges (Fig. 386). Most of them have three-sided solid stems, bearing leaves in three rows. The fruits are nut-like and one-seeded; the embryo, instead of being at one side of the seed as in a grass, is near the base and is entirely surrounded by endosperm. Some "rushes" and so-called "marsh grasses" belong to this family; so do the umbrella plant, and the papyrus which was used in ancient times in the manufacture of paper and from whose name the word paper is derived. 368. Palm Family. This is distinguished from other fami- lies of monocotyledons by the fact that most of its members have woody stems. Many of them are trees, each bearing at its tip a crown of large leaves. Some palms, such as the rattan palm, are climbing plants. Practically all palms are tropi- cal or subtropical. In many species the flowers are borne on a spadix that is enclosed in a spathe. Some have branching flower clusters A (Fig. 387). single flower or- dinarily has six perianth leaves in two whorls of three each, Fio. 387. Flower cluster and fruits of a fan palm (Washingtonia) of southern California. the outer whorl often being distinguished as a calyx, the in- ner as a corolla; there are usually six stamens in two whorls of three each, although in some species the stamens are fewer or more numerous than six; there are three carpels, forming either three separate pistils or one compound pistil. In many species staminate and pistillate flowers are separate and borne either on the same or on distinct plants. The fruit, usually one-seeded, is either a stone fruit as in the coconut, or a berry as in the date. The embryo is at one side of the seed; the seed contains also an abundant endosperm which is often hard. The hard part of the fruit of the date palm is the endosperm; the endosperm of another palm furnishes "vegetable ivory," used in the making of buttons. 482 GENERAL BOTANY The endosperm of the coconut, instead of being hard, constitutes most of the "meat" of the nut. Coconut oil is made from copra, which is the dried meat of the coconut. Palm oil is derived from the fruits of certain species of western Africa and eastern South America. The betel nut, extensively chewed by natives of the East Indies, is the fruit of a palm. Sago is made by washing out the starch which is present in great quantities in the piths of some palms. Among the many other products of palms are fibers of various sorts, such as those from the petioles of the raffia palm; building materials, soap, wax (from the surfaces of stems), and various alcoholic drinks including arrack. 369. Arum Family. Mem- bers of this family are charac- terized by having their flowers crowded on a spadix which is subtended or enveloped by a relatively large, persistent spathe; the spathe is often white or conspicuously col- ored. A familiar native member of the family is the jack-in- the-pulpit or Indian turnip FIG. 388. A, flower cluster of jack-in- the-pulpit. B, staminate flower. C, pistil (constituting a pistillate flower) (Fig. 388). The flowers of this plant are of two sorts, the st, ami.nat, e falowers , . being , borne on the upper part of the spa- dix, the pistillate flowers on the lower part. Not infrequently the flowers of one type abort, so that the functional flowers borne by a particular plant are all staminate or all pistillate. Each stam- inate flower consists of a varied number of short stamens; a pis- tillate flower is but a single simple pistil whose one-chambered ovary contains five or six ovules. The fruit is a scarlet berry with one or two seeds. The aerial shoot which terminates in the spathe and spadix, and which usually bears also two three-parted leaves, is FLORAL TYPES 483 a branch growing from an underground stem. This stem is thick and approximately spherical; like various vegetative parts of many other members of the family, it has an intensely acrid taste. Among familiar plants of the family are the skunk cabbage, sweet flag, and water arum. The arum family is most largely represented in the tropics, and many of the tropical species with showy or oddly shaped spathes are grown in greenhouses and as house plants. Among these are the calla lily, caladium, dracontium, and an- thuriurn. 370. Lily Family. The flower of the hyacinth (Fig. 389) is fairly representative of the flowers of this family. The hyacinth has a raceme with a thick peduncle, each of the spirally arranged flowers being borne in the axil of a small bract. The perianth consists of two whorls of three leaves each, alike in color and shape and united at their bases to form a tube. The outer whorl of perianth leaves may be con- sidered a calyx, the inner whorl a corolla. Near its middle the perianth tube is considerably constricted. Below the con- striction the bases of the fila- ments of the six stamens are FlG - 389 - A6rial portion of a hyacinth united with the perianth tube, plant. but above the constriction each stamen is separate and distinct. Within the perianth tube but entirely separate from it is the pistil. This consists of three carpels and has a three-chambered ovary, a single style, and a three-lobed stigma. Along each line of junction of adjacent carpels their edges are much swollen, and each swollen edge bears a vertical row of ovules. Hence there are six rows of ovules extending through most of the length 484 GENERAL BOTANY of the ovary, and the ovules, together with the edges of the carpels on which they are borne, nearly fill the cavities of the ovary. The hyacinth flower represents a considerably advanced type in the partial union of the staminal filaments with the perianth tube, and in the complete union of carpels. After gametic union, the perianth and stamens are shed; the ovary enlarges greatly and becomes dry, splitting into three compartments each of which con- tains many seeds. Thus the fruit of the hyacinth is a capsule. The flowers of other members of the family are in general similar to that of the hyacinth; the fruits of some are capsules, of others berries. Of the true lilies (members of the genus Lilium), some, such as the Easter lily, tiger lily, and Turk r s-cap lily, have long been cultivated for their flowers. The same is true of many other plants of the family, including the lily of the valley, tulip, orange day lily, and yellow day lily. The greenhouse "smilax" and other species of asparagus are grown for ornamental purposes. Familiar wild plants are the trilliums, Solomon's seal, false Solomon's seal, dogtooth violet, and bellwort. Plants cultivated for food purposes are the asparagus and various members of the genus Allium, in- A cluding onions, garlic, chives, and leeks. few members of the family, including a species of Yucca and the dragon tree (Dracaena), have a special method of secondary thickening, referred to in 46. The family includes also several drug plants and some plants which yield fibers. 371. Orchid Family^In number of species this family, with over 9,000 members, is the largest among monocotyledons; few of its species, however, are abundant and some are very rare. Orchids are characterized by their remarkable bilaterally symmetrical flowers which show the greatest degree of union of floral parts found among monocotyledons. They occupy, therefore, a position among monocotyledons somewhat similar to that of the composites among dicotyledons. The great variety of floral forms in the family seems to represent so many adaptations to insect pollination often to pollination by insects of a certain size and even perhaps of a particular species. The flower of a lady's-slipper (Fig. 390) well illustrates the pos- sibilities of development of an insect-pollinated flower. It has three sepals of which the two lowermost are united, and three petals, one of which, much larger than the other two, has the form of a slipper-like sac open at the top. The opening is partly closed by a FLORAL TYPES 485 The edges of the opening in front of the flap are curved inward. At the bottom of the sac on the inside are juicy hairs that are eaten by insects which make their way into the sac. On the lower side of the flap is the stigma; at either side of the stigma is an anther, whose pollen re- mains together as a sticky mass. A third stamen has no anther. Insects, if not too large, can make their way into the sac in front of the flap, but because of the curved edges of the opening they can not readily escape at the same place. If, however, like some bees, they are sufficiently strong, they can push out through the opening at either side of the flap. In such a case the insect brushes against the anther on that side and carries away its pollen mass. The same insect, entering another flower, brushes against the stigma, where the pollen mass may lodge. Many orchids are grown be- cause of their rarity or for the beauty of their flowers. Not FIG. 390. Lady's-slipper, a member of the orchid family. many are otherwise useful, although the fruit of the vanilla, a tropical American orchid, supplies a well-known flavoring extract, and the dried tubers of some old-world orchids are used, under the name of salep, both as a food and as a drug. Many tropical orchids are epiphytes living high up on the A trunks of trees. few, including the coral root, possess no chloro- phyll; with the aid of fungi in their underground parts they lead a saprophytic life. CHAPTER XXXII INHERITANCE AND VARIATION 372. Inheritance. It is a fact of common observation that off- spring in most respects are similar to their parents, as well as to more distant ancestors. The general rule of resemblance between parent and offspring holds for one-celled as well as for many-celled plants and animals. This rule is implied when it is commonly said that offspring have inherited from their parents such characters as height, color of flower, or a tendency to respond in certain ways to stimuli. It will appear later that the statement that characters such as these are inherited is not strictly accurate. It would be more nearly correct to say that characters possessed by parents have reappeared in their offspring. 373. Independence of Characters. To a considerable extent, distinct characters behave independently in inheritance. That is, an individual plant may show one character that was present in a parent such as tallness, but may not display another character of the same*parent such as flower color. The behavior of characters as something like separate units was demonstrated by the classical experiments of Gregor Mendel (1822-1884). While Mendel was not the first to observe such behavior, he devised a most important method of investigation of the subject in his studies on the common garden pea, the results of which were published in 1866. Mendel first tested -varieties of pea to determine whether they were pure-bred that is, whether they regularly produced offspring like themselves. From among the varieties which proved pure in this sense he selected some which differed one from another in one or more sharply marked characters. These varieties were then crossed ; that is, pollen from flowers of one variety was transferred to the stigmas of another variety. In one experiment a tall variety of pea (six to seven feet high) 1% was crossed with a short variety (% to feet). All the offspring of the cross were tall plants (Fig. 391). These immediate offspring constituted the first filial (Fi) generation. Mendel spoke of the tall character displayed by all the plants of the FI generation as dom- 486 INHERITANCE AND VARIATION 487 inant. The short character, possessed by one parent but not by any of the FI generation, was recessive. When plants of the FI generation, all tall, were self-pollinated, three fourths of their offspring in the second filial (F 2) generation showed the dominant character of tallness, but one fourth dis- played the recessive character of shortness. The recessive character, which seemed lost in the FI generation, had reappeared. When plants of the F2 generation were self-pollinated, the short TaU Parent Short Parent V Fs Generation J?IQ. 391. Diagram illustrating the inheritance of tallness and shortness in Mendel's experiments with peas. ones produced only short offspring (of the F3 generation); one third of the tall F2 plants produced only tall offspring; the remaining two thirds of the tall F2 plants produced, like their parents of the FI generation, offspring of which three fourths were tall and one fourth were short. In another series of experiments Mendel crossed a variety with purple flowers and one with white flowers. The purple-flower character proved dominant over the white-flower character. In the FI, F2, and F8 generations this pair of characters behaved just as had tallness and shortness in the previous experiments. In all, 488 GENERAL BOTANY seven pairs of characters found in different varieties of pea were similarly tested. Mendel studied also the simultaneous inheritance of two pairs of contrasting characters. When a tall variety with purple flowers and a short variety with white flowers were crossed, all the Fj. generation possessed both dominant characters purple flowers and tallness (Fig. 392). The plants of the F2 generation displayed all possible combinations of the two pairs of characters, and these Tall, Purple-, Flowered Parent _ Short, WhiteFlowered \j Parent Fj Generation TaU. Purple. Flowered HUN TaU, Purple Tall, White V F2 Generation III Short, Purple Short, White/ FIG. 392. Diagram illustrating the inheritance of tallness and shortness, purple and white.flower color, in Mendel's experiments. Solid black rods represent purple-flowered plants; white rods, white-flowered plants. combinations appeared in the proportions that would be expected if the characters of one" pair (purple and white flowers) were trans- mitted independently of those of the other pair (tallness and short- ness). The F2 generation consisted, therefore, of plants of four classes in the proportions: nine tall with purple flowers; three tall with white flowers ; three short with purple flowers ; one short with white flowers. Crosses between varieties differing in three pairs of characters for example, purple and white flowers, plump and wrinkled seeds, yellow and green cotyledons gave corresponding but of course more complicated results. The distribution of the characters of any pair among plants of the F2 generation bore no relation to the way in which the characters of any other pair were distributed; INHERITANCE AND VARIATION 489 consequently there appeared in this generation different classes of individuals, in proportions that could be calculated in advance, possessing every possible combination of the characters of the grandparents. Menders description of his experiments was almost completely overlooked until about 1900. Since its rediscovery, studies similar FIG. 393. Results of crossing a red- and a white-flowered four o'clock (Mi- rabilis). All the FI plants had pink flowers. In the F 2 generation J^ had % red, pink, and }^ white flowers. to his have been made upon very many plants and animals. These studies have shown that in large measure characters behave in inheritance as though they were transmitted separately being, as it were, reshuffled and arranged into varying combinations in each succeeding generation. Often, as in the peas studied by Mendel, a particular character of one parent is dominant in the Fj 490 GENERAL BOTANY generation over a contrasting character of the other parent. For example, red, blue, or yellow flower colors are in most cases dominant over white. Hairiness of stems is dominant over smoothness. Brown eye color in man is dominant over blue or gray. In other instances, however, dominance does not appear; instead, the character that appears in the FI generation is in some A degree intermediate between the characters of the parents. case of this nature is that of a cross between a red and a white four o'clock (Fig. 393), which yielded offspring with pink flowers. When these FI pink-flowered plants were self-pollinated, their offspring in the F2 generation were: one fourth red-flowered, one fourth white-flowered, and one half pink-flowered. The latter pink- flowered plants, like their parents, displayed the hybrid character. But whether the FI generation shows complete dominance of one parental character, partial dominance, or intermediacy, it is still true that both parental characters are in effect separately transmitted. In the case in question, the pink-flowered plants trans- mitted to some among their offspring the capacity to produce flowers like those of the original parents namely, red and white. 374. Inheritance and Chromosomes. It was pointed out in Chapter XII that all inheritance must be by means of structures or substances that are transmitted from parent cell to daughter cell in the course of nuclear and cell division. It follows that off- spring do not literally inherit characters from their parents ex- cepting, of course, the structural and functional characters of the spore or gametes which gave rise to the offspring. Apart from characters belonging to such cell or cells received from the parent or parents, all that the new generation inherits is certain substances, the presence of which In the cells of the offspring makes possible A the development of characters like those of the parent. pine tree does not literally inherit tallness; it inherits certain substances which give it the ability to grow tall. It has appeared also that in the main the substances concerned in inheritance are carried in the chromosomes. There is evidence that substances or bodies (such as plastids) present in the cytoplasm play a part also in the transmission of hereditary possibil- ities. But the role of cytoplasmic structures