Introduction 5 



comparable organic forms under a wide range of environmental condi- 

 tions. 



Another important difference between botanical and zoological ma- 

 terial is concerned with the behavior of individual cells during develop- 

 ment. In animal embryos many cells are relatively free to move about so 

 that certain morphogenetic changes are due to movements of cells or cell 

 groups rather than to differences in relative growth. In plants above the 

 simplest types, on the contrary, cells are almost always attached firmly 

 to their neighbors so that morphogenetic movements have no part in de- 

 velopment. Changes of form are the result of differences in the location 

 or orientation of cell divisions or in the size or the shape to which the 

 individual cells grow. This makes the study of morphogenetic problems 

 somewhat simpler in plants because development leaves a record of its 

 course in the structure of the growing system itself. 



Most plant cells have rather firm walls as compared with animal cells, 

 and the structures that they produce are therefore not as soft and plastic 

 as in many animals. A plant part tends to hold its form rather well and 

 can thus be measured more easily and accurately. Its anatomical struc- 

 ture is also less fluid. Certain organs, such as woody stems and hard- 

 shelled fruits, retain their form when dead and dry and can then be kept 

 for study without the necessity for special preservative treatment. 



Plant material is generally more tractable than that of animals, is easily 

 grown, and lends itself readily to experiment. Because of their stationary 

 habit, plants are more susceptible to changes in environmental influences, 

 notably water and light, than are animals, and the morphogenetic effects 

 of such factors may be studied more easily in them. 



The organization of a plant, too, is much looser than that of most ani- 

 mals. The individual organism is less sharply marked and specific, and 

 its powers of regeneration are far greater. Its structural plan is simpler, 

 for the stationary habit of most plants renders unnecessary several organ 

 systems found in animals, notably a digestive tract, excretory organs, 

 musculature, and a nervous system. Morphogenetic problems can there- 

 fore be studied in plants uncomplicated by the physiological complexi- 

 ties inseparable from animal life. The absence of a nervous system, which 

 has such an important role in animal development, is of particular ad- 

 vantage, for developmental processes in plants are under the control of 

 relatively unspecialized protoplasm, and they may thus be studied most 

 directly and at their simplest level. There is no reason to believe that 

 the fundamental phenomena of the development of form are not as 

 manifest in these relatively simple systems as in the more complex ones 

 of animals. 



What is now called morphogenesis came first into prominence in the 

 late decades of the nineteenth century after the early enthusiasm over 



