THE VASCULAR CRYPTOGAMS 107 



different as possible both from Ferns and Lycopods, 

 and this applies both to vegetative structure and 

 spore-bearing organs. Evidently the Horsetails form a 

 perfectly distinct family by themselves. As mentioned 

 above, this family was once, in remote geological ages, an 

 extensive and varied one. Many of its members not 

 only grew into trees, but had the same mode of secondary 

 growth by means of cambium, which is now almost 

 entirely limited to Dicotyledons and Gymnosperms. 

 Their fructifications showed a great variety, some few 

 resembling those of Equisetum, while most of them were 

 more complicated, and several produced spores of two 

 kinds. In fact we can form a much better idea of the 

 Family Equisetineae from the study of its extinct members, 

 than from that of the small remnant which has survived 

 to our own times. 



We have now come to the end of our types of 

 Vascular Cryptogams, and may very briefly sum up the 

 characters of this great and ancient Sub-kingdom of 

 plants. They are quite easily characterised as plants 

 with a clear alternation of sexual and asexual generations, 

 each of which leads a more or less independent life, the 

 asexual stage always being much the more highly de- 

 veloped of the two. The fertilisation by means of 

 spermatozoids, which sometimes occurs even among the 

 Gymnosperms, is here a constant character. The hetero- 

 sporous Vascular Cryptogams come nearest to the Flower- 

 ing Plants, as was fully explained in our chapter on 

 Selaginella, which is the only heterosporous type which 

 we have had space to describe. Heterospory, however, 

 is by no means limited to the Lycopod series ; it occurs 

 also among Ferns (in the widest sense), and, as we have 

 already pointed out, among the fossil Equisetineoe. We 



