1909] 



LEAVITT—HOMOEOSIS IN PLANTS 



63 



Further, if entropic homoeosis has given us decompounding widely 

 in ferns, we shall expect to find species so related to each other that 

 some represent the original types and others the homoeotic deriva- 

 tives. We shall look for, first, the simply pinnate types; secondly, 

 types in which the pinnae match the fronds of type 1; and perhaps 

 thirdly, a type in which the fundamental plan is worked out in the 



pinnules. If the simple type has two 

 modes, as above described for Polystichum 

 acrostichoides, we may expect to find 

 either or both represented in the more 

 complex patterns. 



Anyone having access to a collection 

 of Polystichum may see this projected 

 scheme filled out by existent species. 

 First we have the simply pinnate forms, 

 such as P. acrostichoides, P. munitum, 

 P. lonchitis; in which we have two frond 

 patterns, la being the fashion with more 

 or less oblong, non-auriculate pinnae, 

 probably historically earlier, seen in a 

 pical regions of sterile fronds; and 2b 

 Fig. 19.— Youthful frond being the characteristic plan, the pinnae 



and pinna from adult frond of 

 Polystichum angulare prolif- 



erunu 



fi 



falcate-auriculate. Then we find many 

 species with pinnae frondescent in fac- 

 simile of pattern ia or ib; as P. cali- 

 and P. Lemmonii (id), P. aculeatum (lb), and P. 



sometimes ia); and others, a considerable 



BramU {fig. I0 , lb% ^ 



number of species. Finally, frondescence of the second degree 

 ls weU represented in P. 



segments) 



In P. 



tripinnatum {ia, in the ultimate 

 pulinum the (supposititious) homoeosis is 



complete, the proximal region of the pinnae alone being seg- 

 mented ( Ia ) , the distal merely lobed. 



pin C find Species so mudl alike (except that one of two is simply 

 ae, the other decompound) that one appears to have been 



Pound) 



. from 

 iviparum 



as the West 



ecom 



n Europe P. aculeatum and P. lonchitis stand in a similar 



