332 



Transactions. 



B. LEAF. 



(i.) Leaf -form. 



Each adult plant may be said to have a typical leaf -form, but this is not 

 constant. Leaves are alternate, petiolate, coriaceous, upper surface dark 

 green and slightly glossy, under surface pale green or yellowish-green and 



dull, 4-7*5 cm. long, ovate, oblong-ovate, 



or ovate-lanceolate, acute, margin entire; 



petiole 1-2 cm. long. Besides any one of 



these forms, any individual may bear at 



the base of lateral shoots leaves which 



exhibit many juvenile leaf-forms, the 



most common being orbicular-ovate and 



lanceolate or linear with 



2-6 rounded lobes on each 



side. 



The leaves of seedlings 

 present a remarkable di- 

 versity in shape, and at 

 first no connection be- 

 tween the forms is ap- 

 parent. But in a careful 

 study L. Cockayne de- 

 monstrated that there is a definite pro- 

 cess of development and change : " This 

 complexity arises from the fact that 

 there are two distinct types of leaf — a 

 primary short, broad leaf, and a secondary long 

 and narrow one. Between these two there are 

 all kinds of intermediates, and, moreover, ' re- 

 version shoots ' freely occur, thus bringing 

 primary leaves quite out of their proper place 

 in the sequence [see fig. 5]. The leaves which 

 succeed the cotyledons are certainly variable 

 in size and shape, biit they are always of what 

 may be called the short, broad type. Some- 

 times they are quite small and almost circular, 

 at other times various varieties of oblong pre- 

 dominate. The next phase of development is 

 an increase in length and narrowing of the base 

 of the lamina, so that in the most extreme 

 cases a well-marked spathulate leaf is the re- 

 sult. Then the circular leaf-apex of this latter 

 lost, and the second leaf-form, a long and 



is 



Fig. 5. — Parsonsia heterophylla 

 Seedling. 



narrow leaf, comes into being. This second 

 stage persists for some considerable time — i.e., 

 there is a prolonged juvenile form — but sooner 

 or later, when by the twining of the ever-length- 

 ening stem round its support the bright light 

 is gained, the adult and third form appears, the leaves large and broad, 

 and of a more or less oblong character" (Cockayne, 1908, pp. 486-87). 



The direction in which seedUng-leaves face exhibits to a marked extent 

 •-.he arrangement of leaves so as to receive a maximum amount of light. 



