[72 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



modifications, which are often homoplastic; but underlying these there is 

 the leaf-trace of the Osmundaceous type. 



The Pinna-trace 

 If the theory of leaf-architecture given in Chapter V be correct, and if 

 all the more complex leaf-forms are ultimately referable in origin to dicho- 

 tomy, then the branching of the vascular supply at a forking should give the 

 clue to origin of the supply to the pinna. A good test case will be the regular 

 dichotomous forking of a leaf of Schizaca. A series of transverse sections 

 below a fork of the leaf of 5. dicJiotoina will show how the single meristele here 



Fig. 1 66. Successive sections of the 

 leaf-stalk of Schizaea dichotoina, 

 showing dichotomy of the meri- 

 stele. (xi2.) The adaxial side 

 is uppermost. 



of 



Fig. 167. i, ii, iii, dia 

 grams illustrating the 

 marginal type 

 pinna-supply, as in 

 Pteris umbrosa, R, 

 Br. (After Davie.) 



Fig. 168. Diagrams illus- 

 trating the extra-mar- 

 ginal type of pinna- 

 supply, as in Dryopteris 

 vivipara (Raddi), C 

 Chr. (After Davie.) 



divides into two approximately equal parts by a median constriction (Fig. 

 166). Various degrees of inequality of these parts may be seen in passing 

 from one specimen to another. A sympodial development naturally results 

 in inequality of the branches, and the pinna-trace will be the smaller as a 

 rule. The constriction would then nip off laterally a smaller from a larger 

 portion of the meristele. In many Ferns, and especially in those where 

 the leaves are relatively small, the pinna-trace is still isolated in this way. 



