336 



BOTANY OF THE LIVING PLANT 



corrugated columns. Their strength is further increased according to 

 size and age by the development of masses of sclerotic, adventitious 

 roots, matted together to form a thick investment to the original trunk, 

 and adding to its stability by a method comparable mechanically 

 to a cambial thickening, though quite different in origin. But such 



mechanical provision for increase in size 



are only partially effective. There is 

 no evidence that Ferns ever ranked 

 among the largest of living Plants, 



Many Ferns increase in number by 

 vegetative propagation. This may follow 

 simply on continued growth and 

 branching, as in Pleridium, where the 

 rhizome forks frequently. Whenever 

 progressive rotting extends from the 

 base beyond a branching, the two 

 apices grow on as independent plants. 

 In this way the Bracken multiplies 

 habitually. In Nephrodium buds are 

 formed near the bases of the leaves in 

 old plants. Again, as rotting proceeds 

 from the base, these buds become 

 isolated, and root themselves as new 

 individuals (Fig. 267, B, C). In other 

 Ferns, as in the various species of 

 Asplenium so commonly grown in 

 dwelling rooms, buds or bulbuls arise 

 on the lamina. Being very lightly 

 attached to the leaf they are readily 

 shed, and root themselves indepen- 

 dently in the soil. In some cases 

 vegetative buds may replace the sori 

 (Fig. 277). Such vegetative propa- 

 gation of the Fern-Plant is a mere repetition of the Sporophyte 

 generation. But sooner or later the Fern-Plant bears the spores, 

 which start the alternate generation. 



The spores are produced on certain leaves of the mature plant 

 which are therefore called sporophylls, to distinguish them from those 

 which are only nutritive. In Neplirodiiim nutritive lea^TS and 

 sporophylls are alike in outline. The young plant only produces the 

 former. But the leaves of older plants bear on their lower surface, 



Fig. 277- 

 A pinnae of a Fern (Woodwardia) 

 showing many sporophytic buds on the 

 upper surface. They correspond Ln position 

 to sori on the lower surface, wlriclr are 

 ahorti\-e. 



