—ii5— 



organs of attachment are very common among ferns and 

 other plants, and it is doubtless through roots clinging by 

 such means that these water stores have been evolved. 

 Obviously, too, the deepest roots in every mass of this 

 kind still fasten the plant to its support, but that more 

 than the deepest layer in the mass, which is sometimes 

 two centimeters thick, can serve in this way, is, of course, 

 impossible. There is every gradation from these thick 

 pads down to those so thin that they may serve for at- 

 tachment alone, as is the case with Polypodium macro- 

 phyllum. 



Asplenium epiphyticum has roots of two kinds: those 

 of unlimited length, positively geotropic, forming a jacket 

 around the stem, diarch, flanked by sclerenchyma, un- 

 branched, with hairs along the sheltered side, and roots 

 two or three centimeters long, slightly negatively geo- 

 tropic, freely branching, closely appressed to the sup- 

 port, clinging by copious hairs, of similar structure to 

 the preceding, but with more sclerenchyma. These latter 

 are the clinging roots. The former under favorable cir- 

 cumstances will reach the ground and branch. A. scan- 

 dens likewise has roots of two kinds. The bracing roots 

 of Nephrolepis are very familiar objects. Those of sev- 

 eral species of Diplazhim are very stiff and somewhat 

 spreading above the ground. 



The principles underlying the adaptations of the re- 

 productive structures of ferns are very simple. The 

 sporangia must be protected during their development 

 against injury by dessication or otherwise; the mature 

 spores must dry thoroughly enough to be easily and 

 well scattered, and the drying of the spore must not in- 

 volve too great a dessication of the frond. The struc- 

 tures found in ferns are a compromise between these 

 rather antagonistic principles. Ferns almost always pro- 

 tect their sporangia at the same time that they avoid 

 interference with the illumination of the assimilating 

 organs by restricting the former to the nether surface of 



