8o 



THE STRUCTURE OF ECONOMIC PLANTS 



has proposed two more inclusive terms to be applied in describing 

 the early stages. According to his concept, the apex of the shoot 

 consists of one or more peripheral layers which perpetuate them- 

 selves by continued division, collectively termed the tunica. Within 

 the tunica is the inner core of the growing point which he has 

 designated the corpus. The essential distinction between the two 

 regions is based upon differences in their manner of cell division. 

 The cells of the tunica divide anticlinally to perpetuate its layers, 

 and in periclinal planes only during leaf or bud formation. The 



cells of the corpus, on the other 

 hand, divide in all planes and 

 thus increase the volume of the 

 growing point. These terms have 

 been rather generally adopted in 

 recent literature on leaf develop- 

 ment, and their use appears to be 

 logical, since it is now established 

 that periclinal divisions may occur 

 in the peripheral or dermatogen 

 layer as well as in those which 

 underlie it. Under this system of 

 nomenclature, the number of layers 

 of the tunica is variable, depending 

 upon the plant under considera- 

 tion. It commonly consists of 

 one, two, or three layers. In 

 some plants, leaves may arise 

 exclusively from the tunica; in 

 others the internal tissues of the leaf are derived from the corpus; 

 and an intermediate condition may exist in which the cells of 

 both the tunica and corpus are involved in the differentiation of 

 the leaf. 



In the development of the primordium, the first evidence of its 

 differentiation is associated with the formation of small aggregates 

 of procambial cells. The occurrence of these zones of meristematic 

 tissue precedes the development of any actual protuberance, and 

 results in a transverse expansion of the stem apex to form what 

 Louis (x6) has designated as foliar buttresses, confirming the theories 

 of Gregoire (^3, 14). (Fig. 30.) From these buttresses, the 

 primordia emerge, indicating the intimate relationship between 



Fig. 30. The apical region of two pri- 

 mary shoots of Zea showing apex of a 

 very small vegetative cone from which the 

 leaves b, b', b", and b'" arise as multi- 

 cellular protuberances which soon sur- 

 round the stem. In the axil of the leaf, 

 b", is a rudimentary bud. (After Sachs, 

 Textbook of Botany, Clarendon Press.) 



