DEVELOPMENT OF LEAVES. 



469 



matter of fact where organs of leaf-like nature are interpolated between organs already 

 existing, so that the youngest are no longer next the apex. Payer has detected 

 such cases in a large number of flowers where two or more circles of stamens (that 

 is, metamorphosed leaves) are present, as,- for example, in Didamnus and Geranium. 

 He shows that the second circle of stamens arises on the receptacle outside the first, 

 and therefore further from the apex or centre of the flower. Of course in these and 

 similar cases the question might be raised whether the true embryonic formative 

 tissue is not always perhaps to be sought only at the place where the youngest 

 organs arise— a place which indeed need not always be situated at the apex. 

 However, further pursuit of this question would involve us in difficulties with which 

 I need not trouble the reader of this book. 



The leaves arise at the growing-point of the shoot without exception as super- 

 ficial outgrowths. This does not mean that only the outermost layer of tissue of the 



FIG. 306. — Apical region of two main shoots of Zea Mais. 

 s growing-point consisting of very small cells, from which the 

 leaves V b" b"' proceed as multicellular protuberances which 

 soon embrace the stem and envelope it and the young leaves 

 like a hollow cone. In the axil of the third-youngest leaf h' 

 the youngest branch-rudiment is seen as a rounded protu- 

 berance. 



Fig. 307.— Growing-point at the end of the 

 leaf-shoot of Hippjcris (magnified). 



growing-point, is employed for the formation of the leaf ; but the expression signifies 

 that this layer always takes part in the origin and growth of the young leaf. As 

 a rule, however, layers of tissue situated deeper also co-operate in the formation 

 o£ the leaf. It may thus be stated as the characteristic point that, in the origin of 

 a leaf, complete continuity of the homonymous layers of tissue of the leaf and of the 

 shoot-axis exists from the first, as is clear at once from the diagram Fig. 305, where 

 A is the apex of the growing-point, which, like all growing-points which produce 

 leaves, possesses the confocal structure described in the preceding lecture. B, C, 

 and D, mark various stages in the origin of a new leaf, and the cells and periclinal 

 walls denoted by Roman and Arabic numerals show how the individual cell-layers 

 of the growing-point take part in the origin of a new leaf. The letters a, b, c, x, 

 and J/, in the figure, show how the existing anticlines and periclines are displaced 

 in these processes of growth. Only when the leaf-rudiment has attained a certain 



