Meristems 89 



own. Such organs provide an opportunity for a study of plant develop- 

 ment which has been somewhat neglected in favor of the more sharply 

 limited meristems in apex and cambium. Because of the more diffuse 

 character of their growth, a study of these determinate organs will prob- 

 ably throw more light on the development of form than can be gained 

 from those of indeterminate growth. 



How, we may ask, does a determinate organ grow? Is it through the 

 activity of localized groups of dividing cells, as in the axes, or by un- 

 realized, interstitial growth, as in most animals? The fact is that both 

 methods are usually employed. 



The determinate organ which has been most extensively studied is the 

 leaf. The first step in its development is the appearance of a small swell- 

 ing just below the dome of the shoot meristem. This grows into a leaf 

 primordium and finally, through a series of developmental steps, into a 

 mature leaf (p. 187). As to just how much of the meristematic tissue 

 actually takes part in forming a leaf primordium, there seems to be con- 

 siderable variability among different groups of plants. Rosier (1928) 

 reports that in wheat only the outermost layer (dermatogen) is con- 

 cerned. This pushes out and then pulls together from all sides to meet in 

 the center, like a collapsing glove finger, so that the whole leaf grows 

 from this one layer. Schwarz (1927), on the contrary, found that in 

 Plectranthus and Ligustrum the first two layers produced the entire leaf, 

 and this part of the meristem he termed the phyllogen. Most other 

 workers ( see Foster, 1936 ) have found that tissue below the second layer 

 also contributes often to the formation of the young leaf, particularly the 

 veins. Whether this is simply tunica or both tunica and corpus depends 

 on the extent of layering and seems not to be important. 



Critical evidence in this problem is provided by a study of leaf pri- 

 mordia formed by periclinal chimeras (p. 268). Here one or two of the 

 outer layers come from one of the graft partners and the rest from the 

 other. In chimeras between nightshade and tomato the tissues from each 

 can be distinguished by the fact that in nightshade the cells are much 

 larger. Here Lange ( 1927 ) was able to show that although a leaf primor- 

 dium in this chimera was formed chiefly from the two outer layers the 

 third layer also contributed to it. In periclinal chimeras between forms of 

 Datura stramonium differing in number of chromosome sets (and thus in 

 cell size) Satina, Blakeslee, and Avery (1940) observed the same thing, 

 as did Dermen ( 1947« ) in cranberry. In all these cases the third layer 

 gave rise to the vascular tissue of the leaf. 



The way in which the primordium develops into the leaf also differs 

 considerably in different forms. In fern leaves, growth of the lamina is 

 largely determined by an apical cell resembling that in the shoot and 

 root (p. 58). In many higher plants the early growth of the primordium 



