120 SINNOTT 



character of the structure which develops from it. It is the whole meristematic 

 mass that behaves as an orderly developmental unit. 



Studies of determinate meristems, as in the growth of leaf and fruit, and 

 of lateral ones, especially the vascular cambium, offer a wide and little- 

 exploited field for research. Intensive experimental work on meristems of all 

 kinds is producing a new sort of experimental embryology, quite different 

 from that in zoology but destined to be very fruitful. 



The relation of the character of individual cells to morphogenetic proc- 

 esses is a problem almost as old as the cell theory itself. It can readily be 

 examined in plant material since the cell here has a firm wall, usually is 

 closely attached to its neighbors, and maintains a relatively constant size and 

 shape. The fact that in most cases the size of a plant organ is related to the 

 number rather than to the size of its constituent cells may be taken as evi- 

 dence that the cell is not the basic morphogenetic unit. This conclusion, for 

 which there is much other evidence, is of great theoretical importance if 

 true. In some cases, of course, notably in polyploid forms, organ size is re- 

 lated to cell size. Cell-size differences often reflect environmental influences, 

 also, as in etiolation and physiological dwarfing. 



The shape of cells varies greatly and is a major aspect of differentiation. 

 The origin of these different shapes is a morphogenetic problem in itself, in- 

 volving polarity, geometry, and the fine structure of the cell wall. In most 

 cases, however, cell shape has little influence on organ shape and is relatively 

 unimportant at higher levels of development. 



In some cases the plane of cell division has a definite relation to the direc- 

 tion of growth and it is often closely concerned with differentiation. The in- 

 fluence of external factors such as light, pressure, chemical gradients, and 

 electricity on plant form can often be studied most directly through their 

 effect on the plane of cell division. These facts make it clear that a study of 

 cellular characters is by no means unimportant in developmental problems. 

 The simplest types of plants, notably some of the algae, offer a particularly 

 attractive field for work of this sort. 



In discussing problems in plant morphogenesis above the cellular level 

 one can do little more in brief space than point to some of the promising 

 areas for research. 



One of these is correlation, which in a sense is the problem of organization 

 itself. Much work has been done on the relation of one structure to another, 

 as of the terminal bud to buds below it; on the relative sizes of plant organs, 

 such as root to shoot and cell to organ; and on the relative dimensions of 

 growing plant parts — all these both in normal development and under various 

 experimental conditions. Some of these correlations may be called nutritive, 

 as where root growth depends on food production in the shoot; stimulatory, 

 as where root growth is initiated by auxin, produced in a bud or otherwise; 

 inhibitory, as where one structure inhibits the growth of another, as a termi- 



