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Biological organization is to be seen most distinctly in bodily development. It is obvious that, to produce an individual with a specific form and structure, growth must be more rapid in some directions than in others and must form tissues and organs of different character in different places. Embryology has shown how precisely the activities in one part of the developing individual are related to those in every other part. Few happenings in nature are as fascinating to watch as the unfolding and morphology. More commonly and properly, however, it includes, in addition to a discussion of purely descriptive facts as to the origin of form, a study of the results of experimentally controlled development and an analysis of the effects of the various factors, external and internal, that determine how the development of form proceeds. In other words, it attempts to get at the underlying formativeness in the development of organisms and especially to reach an understanding of the basic fact of which form is the most obvious manifestation-biological organization Workers with plants have a number of advantages, however. In plants, permanently embryonic regions, the meristems, are available for study. At the tip of shoot and root and in the cambial layers these are indeterminate and produce new plant structures almost indefinitely. Such meristems are usually numerous or extensive on the same plant so that ample material for the study of development, identical in genetic constitution, is available. Growth and differentiation in the development of a plant are thus continuing processes and not limited to a single and often brief life cycle.Organs such as leaves, flowers, and fruits, which are determinate in growth, pass through a cycle closely comparable to that of individual animals, and morphogenetic problems can also often profitably be studied in them. The fact that they are usually produced in abundance on a single plant is a further advantage, for here the investigator need not be concerned about genetic diversity in his material but can study strictly The Cellular Basis of Growth 33 structures such as tubers and fruits, the greater size is due to an increase in both the number and the size of their cells. Lehmann (1926) found a positive correlation between the size of a potato tuber and that of its cells. Since the increase in cell size was by no means proportional to that in tuber size, it was evident that large tubers have more as well as larger cells. The same relations are found in tomatoes (Houghtaling, 1935). A more detailed study of this problem, in large-fruited and small-fruited races of gourds, was made by Sinnott (1939; Fig. 3-4). Here cell size increases in the young ovary but much less rapidly than organ size, showing that cell division is taking place. During this period there is more increase in cell size in the larger races. The size at which the cells divide steadily increases. At about the time of flowering, however, division in most of the young fruit ceases, first in the central region and then prog...

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It is generally recognized that during the course of evolution among vascular plants certain rather definite organs or regions of the body have changed much more slowly than others and hence retain many ancient characters which have been lost elsewhere. One of the most important tasks before the student of comparative plant morphology is to determine where these regions are and in what features they are conservative, and thus to aid the phylogenist in picking out those primitive and constant characters on which he may construct a natural system of classification.The reproductive organs, root, young plant, first annual ring, leaf and node in various families have all been shown to be regions which in a greater or less degree are apt to be conservative in their internal or external structure. Among these the anatomy of the leaf, particularly at the node where leaf and stem unite, often retains in a most striking way features which have been lost elsewhere in the plant. The ancient centripetal or ''cryptogamic'' wood has persisted in the foliar bundle of Equisetum (3), the cycads ( 5), and Prepinus (4) after it has disappeared in all other regions, save occasionally in the reproductive axis. The presumably primitive number and arrangement of leaf bundles persists at their point of insertion at the node in the sigillarians, ferns (6), cycads, Cordaitales, Ginkgo and the broad-leaved conifers although it has changed greatly in the petiole and blade. Sinnott, Edmund W. 1914. "Investigations on the phylogeny of the angiosperms. I. The anatomy of the node as an aid in the classification of angiosperms." American journal of botany 1(7), 303-322.