The aerating tissue of the plant performs a threefold function: first, it permits the exchange of gases during photosynthesis; secondly, it permits the entrance of oxygen and the exit of carbon dioxide during respiration; and, thirdly, it permits the exit of the excess of water absorbed by the plant. The above functions are carried on by the stomata, the water-pores, the lenticels, and the intercellular spaces of the plant. The stoma functions as the chief channel for the passage of CO2-laden air into the leaf and of oxygen-laden air from the leaf to the atmosphere. The stoma also functions as an organ of transpiration, since through the stoma a large part of the excess water of the plant passes off into the air.<Callout type="tip" title="Tip">Understanding stomata is crucial for recognizing how plants manage their internal environment.</Callout> In certain plants, the primary epidermis is provided with openings resembling stomata, but unlike stomata the orifice remains open, and instead of being located on the upper or lower surface of the leaf, they are located on the margin of leaves immediately outward from the veins. Water is given off to the atmosphere from these openings. Such an opening is usually designated as a water-pore.<Callout type="important" title="Important">Water-pores differ significantly in their location and function compared to stomata.</Callout> The chief external openings of the epidermis of leaves, of herbs, and of young wood stems are known as stomata. Surrounding the stoma are two cells known as guard cells.<Callout type="risk" title="Risk">Misidentifying or misinterpreting the function of stomata can lead to incorrect conclusions about a plant's health.</Callout> Guard cells differ greatly in form, in size, in arrangement, in occurrence, in association, in abundance (Plates 53, 54, and 55), and in color. The guard cells surrounding the stoma vary in form from circular to lens-shaped. In most leaves, the outline of the guard cells is rounded or has an arched outline; but in a few cases the guard cells have angled outlines.<Callout type="beginner" title="Beginner">For beginners, understanding that guard cells can be various shapes and sizes helps identify different plant species.</Callout> The arrangement of the surrounding cells of the stoma is one of the most important characteristics of the different leaves. As a rule, the number of surrounding cells about a stoma is constant for a given species.<Callout type="gear" title="Gear">Microscopes are essential tools for accurately observing and studying stomata.</Callout> The relation of the stoma to surrounding cells is best shown in cross-sections of the leaf. In powders, the relationship of the stoma to the surrounding cells is readily ascertainable.<Callout type="important" title="Important">Always consider the size and form of the surrounding cells when studying leaves.</Callout> There is the greatest possible variation in the size of guard cells. This fact must always be kept in mind when studying leaves. This variation in the size of the guard cells is clearly illustrated by coca, senna, and deer's-tongue. In coca the stomata are very small; in senna they are larger; while in deer's-tongue the stomata are very large.<Callout type="warning" title="Warning">Ignoring variations in guard cell size can lead to misinterpretations of plant health.</Callout> The width and length of the stoma or opening between the guard cells are of a character which must not be overlooked. Generally speaking, those leaves which have large guard cells will have correspondingly large stomata.<Callout type="tip" title="Tip">Measuring stomatal size can provide insights into plant health and environmental conditions.</Callout> The guard cells usually contain chloroplasts showing various stages of decomposition. In bay-rum leaf the guard cells are of a bright reddish-brown color, but in most leaves the guard cells are colorless.<Callout type="important" title="Important">Color changes in guard cells can indicate environmental stress or disease.</Callout> Lenticels are small openings occurring in the bark of plants. The lenticels bear the same relationship to the stem that the stomata do to the leaves. Lenticels, like stomata, have a threefold function — namely, exchange of gases in photosynthesis, in respiration, and the giving off of water.<Callout type="risk" title="Risk">Misidentifying lenticels can lead to incorrect conclusions about plant health or species.</Callout> Lenticels are macroscopically as well as microscopically important. When unmagnified the lenticels are circular, lens-shaped, or irregular in outline. They are arranged in parallel longitudinal lines or parallel transverse lines, or they are irregularly scattered. The latter is the usual arrangement.<Callout type="important" title="Important">Lenticel patterns can be used to identify plant species and assess their health.</Callout> On cross-sections the lenticel (Plate 57, Fig. 2) is seen to have a central depressed portion made up of loosely arranged cells. Bordering the cavity are typical cork cells.<Callout type="warning" title="Warning">Incorrectly identifying lenticels can lead to misdiagnosis in plant health assessments.</Callout> The size of lenticels will vary according to the type of the lenticel. In studying sections more attention should be paid to the character of the cells forming the lenticels than to the size of the lenticel.<Callout type="important" title="Important">Focus on cell characteristics rather than just overall lenticel size.</Callout> On cross-section, the intercellular spaces (Plate 58) are triangular, quadrangular, or irregular. The spaces between equal diameter parenchyma cells is triangular if three cells surround the space, and quadrangular if four cells surround the space, etc.<Callout type="tip" title="Tip">Understanding intercellular spaces can help in assessing plant health and water management.</Callout> These spaces are in direct contact with similar spaces that traverse the tissue at right angles to its long axis. The branched mesophyll cells of the leaf and aquatic plant parenchyma (Plate 59) are arranged around irregular cavities.<Callout type="important" title="Important">Intercellular spaces play a critical role in gas exchange and water management.</Callout> In leaves and aquatic plants these spaces run parallel to the long axis of the organ. In each of the above cases the cavity is formed by the separation of the cell walls. There is still another type of irregular cavities which is formed by the dissolution or tearing apart of the cell walls. Such cavities are found in the stems and roots of many herbs.<Callout type="risk" title="Risk">Misunderstanding intercellular spaces can lead to incorrect assessments of plant health.</Callout> The pith cells in the stems of many herbs become torn apart during the growth of the stem, with the result that large irregular cavities are formed. These cavities are usually filled with circulatory air.<Callout type="important" title="Important">Large intercellular spaces can indicate healthy plant growth and efficient gas exchange.</Callout> In the stems of conium, cicuta, angelica, and other larger herbaceous stems the pith separates into layers. When a longitudinal section is made of such a stem it is seen to be composed of alternating air spaces and masses of pith parenchyma.<Callout type="tip" title="Tip">Observing intercellular spaces can provide insights into plant health and growth patterns.</Callout> The intercellular spaces are very large in leaves where enormous quantities of carbon dioxide are vitalized in photosynthesis. In the rhizome of calamus and other aquatic plants the intercellular spaces are very large.<Callout type="important" title="Important">Large intercellular spaces in aquatic plants indicate efficient gas exchange.</Callout> The cells of these plants are arranged in the form of branching chains of cells which thus provide for large intercellular spaces. The cells of the middle layer of flower petals, like the mesophyll of leaves, is loosely arranged owing to the peculiar branching form of the cells.<Callout type="important" title="Important">Loosely arranged cells facilitate efficient gas exchange in various plant parts.</Callout> Seeds and fruits contain, as a rule, few or no intercellular spaces.
Key Takeaways
- Stomata are crucial for gas exchange in plants during photosynthesis and respiration.
- Guard cells surrounding stomata vary greatly in form, size, and arrangement.
- Lenticels in bark serve similar functions to stomata but are located differently.
- Intercellular spaces play a vital role in plant health and efficient gas management.
Practical Tips
- Use microscopes to accurately identify and study stomata for better understanding of plant health.
- Recognize the importance of guard cell size and form when assessing plant species or environmental conditions.
- Understand that lenticels can be used to identify plant species and assess their overall health.
Warnings & Risks
- Misidentifying or misinterpreting stomata, lenticels, or intercellular spaces can lead to incorrect conclusions about a plant's health.
- Ignoring variations in guard cell size can result in misinterpretations of plant health.
- Incorrectly identifying lenticels can lead to misdiagnosis in plant health assessments.
Modern Application
Understanding the aerating tissue in plants, as detailed in this chapter, is still relevant for modern survival preparedness. While direct application may not be immediate, knowledge of how plants manage their internal environment can inform sustainable practices and emergency situations where natural resources are limited.
Frequently Asked Questions
Q: What are stomata and what do they do?
Stomata are the chief external openings in leaves, herbs, and young wood stems. They permit the exchange of gases during photosynthesis and respiration, as well as the exit of excess water through transpiration.
Q: How can one identify different types of stomata?
Guard cells surrounding stomata vary greatly in form, size, and arrangement. For example, coca leaves have very small stomata, while senna leaves have larger ones, and deer's-tongue leaves have very large stomata.
Q: What are lenticels and how do they function?
Lenticels are small openings in the bark of plants that serve similar functions to stomata. They facilitate gas exchange during photosynthesis, respiration, and water release from the plant.