12 2 SINNOTT 



ably due to geometrical relationships between the primordia of lateral struc- 

 tures in meristems of different sizes and different degrees of crowding. Phyl- 

 lotaxy has always intrigued morphologists since it is a place where organic 

 form can be interpreted in mathematical terms. At least it is clear that sym- 

 metry of this sort, simple though it be, is quite different from the rigid sym- 

 metry of crystals, to which it has often been compared. It may perhaps be 

 based on a fundamental spiral tendency in living stuff itself which is evidenced 

 in traits as different as the spiral pattern of cellulose micelles in the cell wall 

 and spiral growth movements and nutation. 



The distortion of radial symmetry to dorsiventral or other types has often 

 been studied. In some cases this is due to the direct effect of an external fac- 

 tor like light or gravity; in others it has become fixed through evolutionary 

 changes. Nowhere can the relation between environmental and genetic con- 

 trol of organic form be studied in simpler manifestations than in these two 

 types of dorsiventrality in plants. The polarity and symmetry that are in- 

 herent in the structures protoplasm builds and that are so vividly expressed 

 in plants seem to be the basis of most organic patterns. 



But a plant is more than a polar axis with symmetrical structure. Its parts 

 are different from one another. The first difference is between the two ends 

 of the polar axis. In the course of development from spore or seed various 

 lateral organs appear along this axis, such as leaves in higher plants. In most 

 cases these are produced in a series of successive members, from juvenile 

 stages through various levels in the adult and culminating in the formation 

 of reproductive structures. The plant thus has a definite life cycle. The suc- 

 cessive multiple parts of which the body is composed often show progressive 

 differences in structure, and evidence is accumulating that there are also 

 physiological differences among them. In animals these various steps in the 

 life cycle are manifest in changes within a single individual, but in plants 

 the steps are spread out among the successively differentiating parts. Plants 

 offer the student of development an advantage in this regard since he can 

 separate the various stages in the process instead of having to follow them 

 in a single unfolding system. 



This system is not an unchanging one in differentiation. The potency of 

 a cell or group of cells is altered as the organism develops. Originally it may 

 be able to produce, by regeneration, an entire individual and is thus totipotent. 

 This ability is reduced little by little until only one fate is left for a cell unless 

 it becomes embryonic again. The reactivity of a cell also changes. The 

 response it will make to a given factor in the environment may be very dif- 

 ferent at successive stages of development. This is related to the rhythmic 

 physiological alterations which the whole plant undergoes. Furthermore, both 

 the potency and the reactivity of a cell may become very different in various 

 parts of the plant as differentiation progresses. 



The ease of regeneration in plants can often be used to test the degrees 



