[CHAPTER XXXVIII.]
THE ORGANIZATION OF THE PLANT.
[I. Organization of Plant Members.][37]
689. It is now generally conceded that the earliest plants to appear in the world were very simple in form and structure. Perhaps the earliest were mere bits of naked protoplasm, not essentially different from early animal life. The simplest ones which are clearly recognized as plants are found among the lower algæ and fungi. These are single cells of very minute size, roundish, oval, or oblong, existing during their growing period in water or in a very moist substratum or atmosphere. Examples are found in the red snow plant (Sphærella nivalis), the Pleurococcus, the bacteria; and among small colonies of these simple organisms (Pandorina) or the thread-like forms (Spirogyra, Œdogonium, etc.). It is evident that some of the life relations of such very simple organisms are very easily obtained—that is, the adjustment to environment is not difficult. All of the living substance is very closely surrounded by food material in solution. These food solutions are easily absorbed. Because of the minute size of the protoplasts and of the plant body, they do not have to solve problems of transport of food to distant parts of the body. When we pass to more bulky organisms consisting of large numbers of protoplasts closely compacted together, the problem of relation to environment and of food transport become felt; the larger the organism usually the greater are these problems. A point is soon reached at which there is a gain by a differentiation in the work of different protoplasts, some for absorption, some for conduction, some for the light relation, some for reproduction, and so on. There is also a gain in splitting the form of the plant body up into parts so that a larger surface is exposed to environment with an economy in the amount of building material required. In this differentiation of the plant body into parts, there are two general problems to be solved, and the plant to be successful in its struggle for existence must control its development in such a way as to preserve the balance between them. (1) A ready display of a large surface to environment for the purpose of acquiring food and the disposition of waste. (2) The protection of the plant from injuries incident to an austere environment.
It is evident with the great variety of conditions met with in different parts of the same locality or region, and in different parts of the globe, that the plant has had very complex problems to meet and in the solution of them it has developed into a great variety of forms. It is also likely that different plants would in many cases meet these difficulties in different ways, sometimes with equal success, at other times with varied success. Just as different persons, given some one piece of work to do, are likely to employ different methods and reach results that are varied as to their value. While we cannot attribute consciousness or choice to plants in the sense in which we understand these qualities in higher animals, still there is something in their “constitution” or “character” whereby they respond in a different manner to the same influences of environment. This is, perhaps, imperceptible to us in the different individuals of the same species, but it is more marked in different species. Because of our ignorance of this occult power in the plant, we often speak of it as an “inherent” quality.
Perhaps the most striking examples one might use to illustrate the different line of organization among plants in two regions where the environment is very different are to be found in the adaptation of the cactus or the yucca to desert regions, and the oak or the cucurbits to the land conditions of our climate. The cactus with stem and leaf function combined in a massive trunk, or the yucca with bulky leaves expose little surface in comparison to the mass of substance, to the dry air. They have tissue for water storage and through their thick epidermis dole it out slowly since there is but little water to obtain from dry soil.
The cucurbits and the oak in their foliage leaves expose a very large surface in proportion to the mass of their substance, to an atmosphere not so severely dry as that of the desert, while the roots are able to obtain an abundant supply of water from the moist soil. The cactus and the yucca have differentiated their parts in a very different way from the oak or the cucurbits, in order to adapt themselves to the peculiar conditions of the environment.
When we say that certain plants have the power to adapt themselves to certain conditions of environment, we do not mean to say that if the cucurbits were transferred to the desert they would take on the form of the cactus or the yucca. They could do neither. They would perish, since the change would be too great for their organization. Nor do we mean, that, if the cactus or yucca were transferred from the desert to our climate, they would change into forms with thin foliage leaves. They could not. The fact is that they are enabled to live in our climate when we give them some care, but they show no signs of assuming characters like those of our vegetation. What we do mean is, that where the change is not too great nor too sudden, some of the plants become slightly modified. This would indicate that the process of organization and change of form is a very slow one, and is therefore a question of time—ages it may be—in which change in environment and adaptation in form and structure have gone on slowly hand in hand.
690. Members of the plant body.—The different parts into which the plant body has become differentiated are from one point of view, spoken of as members. It is evident that the simplest forms of life spoken of above do not have members. It is only when differentiation has reached the stage in which certain more or less prominent parts perform certain functions for the plant that members are recognized. In the algæ and fungi there is no differentiation into stem and leaf, though there is an approach to it in some of the higher forms. Where this simple plant body is flattened, as in the sea-wrack, or ulva, it is a frond. The Latin word for frond is thallus, and this name is applied to the plant body of all the lower plants, the algæ and fungi. The algæ and fungi together are sometimes called thallophytes, or thallus plants. The word thallus is also sometimes applied to the flattened body of the liverworts. In the foliose liverworts and mosses there is an axis with leaf-like expansions. These are believed by some to represent true stems and leaves; by others to represent a flattened thallus in which the margins are deeply and regularly divided, or in which the expansion has only taken place at regular intervals.
In the higher plants there is usually great differentiation of the plant body, though in many forms, as in the duckweeds, it is in the form of a frond. While there is a great variety in the form and function of the members of the plant body, they are all reducible to a few fundamental members. Some reduce these forms to three, the root, stem, leaf; while others to two, the root, and shoot, which is perhaps the best primary subdivision, and the shoot is then divided into stem and leaf, the leaf being a lateral outgrowth of the stem, and can be indicated by the following diagram:
Plant body···· |  | Stem. | ||
| Shoot···· | |||
| Leaf. | ||||
| Root. |