94 



tamorphosed into green leaves. In the flower the axile forma- 

 tion seems to disappear entirely and presents itself as the re- 

 ceptacle. The transformed leaves now stand no longer one 

 above another, but side by side, and are therefore developed 

 nearly cotemporaneously. 



There are three essential points in the ascending growth, in 

 the development of which the whole formative principle is in 

 action ; node, leaf, and internode. If we inquire into the ori- 

 gin of leaves, we find a thickening of the axis and a conver- 

 gence of its vessels in the node, just beneath the lateral inser- 

 tion of the leaf. Above a node the vessels again resume 

 their regular ascending course ; they return to perfect order 

 and symmetry, until at the end of the internode they come to 

 a new node and foliaceous formation, and thus several inter- 

 nodes stand one above another in the axile development. The 

 formation of each collateral axis is also stated to be preceded 

 by that of nodes and leaf. 



Second Lecture. Leaves with distinct sheath processes, 

 with petiole and with the peculiar foliaceous expansion, are 

 regarded by M. von Martius as perfect leaves ; however, all 

 leaves do not possess these three parts, but sometimes the 

 one, sometimes the other exhibits a predominant develop- 

 ment. From these differences three kinds of leaves may be 

 distinguished : sheath leaves (Coelophylla), petiole leaves 

 (Steleophylla), and surface or broad leaves (Placophylla). The 

 succession of the green leaves from the root to the leaves of 

 the inflorescence is of high importance for the history of 

 the development of the plant. In general the formation of 

 leaves commences with Coelophylla, further upwards on the 

 stem both the other dimensions of the leaf occur, and when it 

 comes to a ramification of the axis one or the other of these 

 stages of the leaf is again lost. These changes are still 

 more remarkable when a flower proceeds from the axis in- 

 stead of a leaf-bearing branch. These are the bracts, called 

 ante-leaves or bearing leaves, the functions of which still co- 

 incide with those of the common leaves. But the changes 

 which the petals and sexual leaves exhibit are far more de- 

 cided in form and colour. At the metamorphosis of the 

 leaf into stamen the formation of the pollen proceeds in its 

 interior. This metamorphosis takes place in the upper or 



