Parsons : Flowerless Plants and their Habitats. 



21 



difficulty is met with iu defining the limits of the several orders : a 

 lichen, a fungus, and a seaweed look very distinct, but intermediate 

 forms are found connecting them so closely that it is difficult to say 

 where one order ends and another begins. 



The cryptogams are classified into two divisions — the acrogens, 

 which have a distinct stem and leaves, and the thallogens, which 

 consist of a frond without any distinction between stem and leaves. 

 Here again we find how hard it is to bind Nature by one rule of 

 classification, for some of the liverworts have not only distinct leaves 

 bat stipules, while others which resemble them so closely in other 

 respects that they cannot be removed from them, have a flat green 

 frond without stem or leaves. On the other hand, the genus 

 Delesseria among the red seaweeds has beautifully formed leaves with 

 stalks and veins, though consisting only of cellular tissue. 



The Acrogens are divided into two groups, according as they 

 contain vessels and woody fibre, or are composed wholly of cellular 

 tissue. The vascular Acrogens comprise the ferns and three or four 

 smaller orders, apparently of little consequence at the present day, 

 but which have played a highly important part in the history of our 

 globe, as containing the plants of which the carbonized remains form 

 our beds of coal, and as being the road by which, if the theory of 

 evolution be true, the vegetable world was developed in its upward 

 progress through the coniferae into the flowering plants. Time will 

 not allow me to do more than glance briefly at the characters of the 

 several orders. Ferns have a stem which in our British species is 

 either a short rootstock or a creeping underground rhizome, but 

 which in many fossil and exotic kinds forms a tall erect trunk, and 

 in some others has a climbing habit. The leaves are large, generally 

 much and elegantly divided, and coiled spirally when young. The 

 fructification is borne at the back of the leaves, or on certain 

 modified leaves ; it consists of clusters of spore cases often covered 

 at first with a transparent membrane. The spore cases in most of 

 our British species burst by means of an elastic ring round the 

 margin, and scatter the spores. This is the non- sexual stage of the 

 plant. The spores develop, not into a fern-plant like that from which 

 they spring, but into a small green heart-shaped frond like a liver- 

 wort, called the prothallus ; this frond bears antheridia and arche- 

 gonia, from the union of which the young fern springs. Thus, while 

 the non-sexual stage may be a lofty tree, enduring for a century, the 

 sexual stage is no bigger than one's finger-nail, and perishes in a few 

 weeks. In certain annual species, however, the prothallus is more 



