494 



BOTANY OF THE LIVING PLANT 



accompany and enclose the flattened meristeles ; and their margins 

 are usually curved outwards, thus securing increased mechanical 

 resistance on the same principle as in corrugated columns. Their 

 strength is further increased according to size and age by the develop- 

 ment 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 mechani- 

 cal provision for increase in size is only 

 partially effective. There is no evi- 

 dence 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 Pteridium, 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 Dryopteris 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. 374, B } C). In other 



Fig. 386. 



A pinna of a Fern (Woodwardia) show- 

 ing many sporophytic buds on the upper 

 surface. They correspond in position to 

 son on the lower surface, which are 

 abortive, and they may be held to be sub- 

 stitutionary growths. 



Ferns, as in the various species of 



Aspleniuvi so commonly grown in 

 dwelling rooms, buds or bulbils arise 

 on the lamina. Being very lightly 

 attached to the leaf they are readily 

 shed, and root themselves independently in the soil. In some cases 

 vegetative buds may replace the sori (Fig. 386). Such vegetative 

 propagation 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 



