Part 2
this regularity of structure is usually wanting, owing to the presence of large air-chambers in the cortex, separated by very thin radial plates of cells. This feature is especially conspicuous in sedges of aquatic or limicolous ‘habit. The piliferous layer is not specially modificd, and is usually B2 4 Plowman.—The Comparative Anatomy and strengthened by a narrow hypodermal zone of more or less sclerotic mechanical tissue. Tannin-sacs are of quite common occurrence in all parts of the root. The primary root arises from the micropylar end of the protocaulome. It is of relatively short duration and little importance. Secondary roots arise endogenously from the rhizome. Rootlets are derived from the rhizogenous pericambium situated just within the endodermal sheath. It is a well-known fact that in the Gramineae this pericambial sheath is in- terrupted at the xylem plates, which lie in actual contact with the en- dodermis. The earlier investigators believed the grasses to be altogether unique in this respect, all other Monocotyledons as far as studied showing a continuous rhizogenous zone. An uninterrupted pericambium has been demonstrated by Van Tieghem (54) in species of Carex and Cyperus, and more recently by Holm (25) in Carex Frasert. However, later studies by Van Tieghem and: Douliot (58), De Bary (12), and Klinge (81), reveal the interesting fact that the majority of the Cyperaceae resemble the Gramineae in the possession of an interrupted rhizogenous pericambium, while a few species have a uniform, continuous pericambial sheath, and still other species show various stages intermediate between these two conditions. Consequently there are two types of origin of rootlets. In the majority of cases, where the rhizogenous zone is interrupted by the smaller xylem- elements, the rootlet is inserted opposite a phloem-strand, and the xylem- elements are derived from the two adjacent xylem-strands, so that the oval base of origin of the rootlet has its longer axis transverse to the main axis of the parent root. On the other hand, in those cases in which the peri- cambium is continuous the rootlets are inserted directly upon the xylem- strands, each rootlet connected with but a single strand, and having the longer axis of its oval base of attachment parallel to the main axis of the parent root. These facts have a certain interest from their bearing upon the relationships of grasses and sedges as expressed by Van Tieghem (56) and others, and to be considered more fully on a subsequent page. In its apical development the root presents no features uncommon to the great majority of the Monocotyledons. As shown by De Bary (12), Van Tieghem (56), Treub (53), and others, the root-cap is developed from a distinct calyptrogenic initial meristem, outside and independent of the dermatogen, so that the piliferous layer presents a smooth contour, traceable under the root-cap quite to the initial region. THE RHIZOME. The great majority of the Cyperaceae grow by a perennial root-stock, which is in some cases long and slender, in other cases extremely short and compact, while other members of the Order present all intermediate Phylogeny of the Cyperaceae. 5 conditions. Even the annual species commonly preserve the rhizomatous habit, the first few internodes of the stem being very short and usually growing more or less obliquely in the soil. Indeed, where the rhizomatous character of the stem-base is not apparent externally, it is always demonstrable from internal structure. Frequently the rhizome bears hard, brown leaves of considerable size at the nodes, but more generally the leaves are reduced to mere scales. After a period of horizontal or oblique growth the rhizome bifurcates to give rise to an aerial stem or culm which is commonly leafy, at least at the base, and which ultimately bears the inflorescence. Not infrequently several aerial axes are given off from the rhizome in a single season’s growth, some of them being only vegetative, while others bear both leaves and flowers. But it more often happens that the number of aerial shoots is very small in each season’s growth, and all of them ultimately develop flowers. Fibrous roots are produced in large numbers from the nodes of the rhizome. In most cases the elongated rhizomes are not of uniform size throughout their length, but consist of swollen nodes and slender internodes, as in Scirpus americanus; or the rhizome may be made up of swollen tuberous portions connected by slender internodes, as in Cyperus esculentus, and to a less marked degree in Scirpus cyperinus, S. atrovirens, S. lineatus, &c.; only rarely does one find the rhizome of uniform size for a considerable part or the whole of its length, as in Scirpus robustus and a few of the Carices. Internally the rhizome consists of a central cylinder which is very rarely hollow in the mature condition, as in Carex scoparia, Cladium mariscoides, Dulichium arundinaceum, &c., but which as a rule is solid or only slightly aerenchymatous, and composed of several to very many fibro- vascular bundles embedded in fundamental tissue and surrounded by a well-defined endodermal sheath (except in Scirpus microcarpus and Cladium mariscotdes), which is either simple or variously reinforced by circular fibres or a sclerotic zone. Outside the endodermis is the very variable cortex, bounded externally by the epiderm. The cortex may be sclerotic in whole or in part, unmodified parenchyma, aerenchymatous in varying degree, or even cavernous as in the upper side of the rhizome of Dulichium. Bordet (6), Palla (35), and others have attempted to utilize the characters of the cortex as diagnostic features of the Cyperaceae, but with very doubtful success, owing to the fact that these features are extremely variable under change of environment. The peripheral portion of the cortex is very generally modified to form a sclerotic hypodermal zone, or at least a series of hypodermal ribs, of which the function is clearly mechanical. The epidermis is not infrequently sclerotic, and rarely covered with a thick cuticle. Tannin-sacs are of common occurrence throughout most rhizomes. With a few rare exceptions among the Carices, starch is everywhere present 6 Plowman.—The Comparative Anatomy and in the rhizome of the Cyperaceae. It is found in especially great quantities in Scirpus robustus and S. americanus, and in the tuberous rhizomes of many species of Cyperus. It has been found that outside the central cylinder all of the characters . are more or less highly susceptible to the influence of environment, often presenting a considerable range of variation within the same species, and consequently possessing comparatively little value from a phylogenetic point of view. Accordingly our attention will be directed more specifically toward the characters of the central cylinder itself. In the seedlings of Scirpus microcarpus the central cylinder presents a condition of affairs strikingly similar to that described and figured by Chrysler (9) for S#z/acina, the only difference being found in the somewhat more numerous leaf-gaps and in the earlier appearance of numerous medul- lary strands, which, however, are almost without exception of the collateral type. Young plants of other species indicate a similar tubular condition of the central cylinder in its earlier stages, followed in later development by the appearance of medullary strands, which in the majority of the Cyperaceae are of the concentric type. Thus it appears from a cursory glance at the seedling of the Cyperaceae that the central cylinder in its earliest stages is a simple siphonostele of the phyllosiphonic type, similar in every essential particular to that of the Dicotyledons. Only in later stages of growth does one find a highly complicated arrangement of stelar struc- ture. It seems very desirable that the developmental history of the Cyperaceae should be more thoroughly studied. The central cylinder of the mature rhizome may be described under two fairly well-marked general types, which are characterized by the disposition of the xylem-elements of the fibro-vascular bundles. In the first of these types, which is by far the more common among the Cyperaceae, all or most of the bundles are of the concentric sort, with the xylem distri- buted more or less uniformly all around the phloem. To such bundles the term ‘amphivasal ’ has been very generally applied, and those Cyperaceae which present this type of rhizome bundle may, for purposes of description, be grouped together as the Amphivasae. Here belong most species of Scirpus, Cyperus, Eleocharis, Ertophorum, Rhynchospora, and Carex. In certain other forms, notably Dulichium, Cladium, Scirpus sicrocarpus, S. americanus, S. robustus, &c., the bundles are of the simple collateral type, with the relatively few xylem-elements disposed on the centripetal side of the bundle. Those forms which present this arrangement may be termed the Centrivasae. . Scirpus cyperinus will serve as an example of the Amphivasae. A part of the central cylinder of this species is shown in Pl. I, Fig. 3, and a part more highly magnified in Fig. 4. It will be observed that the bundles are very numerous and almost exclusively of the amphivasal