190 



SIZE A FACTOR IN STELAR MORPHOLOGY 



[CH. 



If, as the anatomy of the Ferns seems to suggest, actual size is one of 

 the factors determining the form which the stelar tissues take, and that 

 increase beyond certain dimensions tends towards those pecuHarities which 

 are seen in them, then tuberous development should lead to such disintegra- 

 tion. More especially should the change be apparent where the normal part 

 shows a relatively simple stelar structure. A good example of this is seen 

 in the tubers borne upon the protostelic stolons of Nephrolepis (Fig. 182). It 

 has been shown by Lachmann {Thesis, Paris, 1889), and by Sahni {New 

 Phyt. Vol. XV, p. 72, 19 1 6), that the protostele of the stolon expands at the 

 base of the distended tuber. As seen in transverse section it first acquires a 

 central mass of phloem, followed successively by pericycle, endodermis, and 



" A 3 



Fig. 182. Nephrolepis cordifolia. A, stolon bearing a tuber, in which the 

 protostele breaks up into a cylindrical network, contracting again at the 

 apex. 7? = root. (After Sahni.) i?, transverse section of protostelic stolon, 

 (x 5.) C, transverse section of tuber (also x 5) showing ring of meristeles 

 each limited by endodermis. Diameter of stolon, r6mm. Diameter of 

 tuber, ii"omm. 



ground-parenchyma. In fact, it becomes solenostelic. As the base of the 

 tuber expands further the ring breaks up by irregular perforations, as it does 

 in the leafy shoots of many Leptosporangiate Ferns. But here there are only 

 perforations; since no leaves are borne on the stolons there are naturally no 

 foliar gaps. A network of meristeles is thus formed, each limited by a com- 

 plete endodermis, and it is arranged as an expanded ring (Fig. 182, C). At the 

 distal end where the tuber contracts again the network narrows down through 

 stages of condensation the reverse of the previous disintegration. In a given 

 case the diameter of the stolon was i"6mm., and of its protostele •6 mm. 

 The diameter of the tuber was ri cm., and of its ring of meristeles 74 cm. ; 

 that is nearly fifteen times that of the original protostele. It thus appears 

 that, while complete endodermal control is maintained, when the stolon of 



