THE MORPHOLOGICAL COMPOSITION OF PLANTS. 47 



Mosses and Ferns, we find a distinctly marked stem.* Some 

 Acrogens have foliar expansions that are indefinite in their 

 forms ; and some have quite definitely-shaped leaves. Hoots 

 are possessed by all the more developed genera of the class ; 

 but there are other genera, as Sphagnum, which have no 

 roots. Here the fronds are thallus-like, in being formed of only 

 a single layer of cells ; and there a double layer gives them 

 a more leaf-like character — a difference exhibited between 

 closely-allied genera of one order, the Mosses. Equally varied 

 are the developments of the foliar-organs in their detailed 

 structures : now being without mid-ribs or veins ; now having 

 mid-ribs but no veins ; now having both mid-ribs and veins. 

 Where stem and leaves exist, their imperfect differentiation 

 is shown by the fact, that in many cases the stem is covered 

 by an epidermis containing stomata. lior must we omit the 

 similarly-significant circumstance, that whereas in the lower 

 Acrogens, the reproductive elements are immersed here and 

 there in the thallus-like frond ; they are, in the higher orders, 

 seated in well- specialized and quite distinct fructifying 

 organs, having analogies with the flowers of Phsenoganis. 

 Thus, many facts imply that if the phosnogamic type is to be 

 analyzed at all, we must look among the Acrogens for its mor- 

 phological components, and the manner of their integration. 

 Already we have seen among the lower Cryptogamia, how 



* Sclileiden, who chooses to regard as an axis, that which Mr Berkeley, with 

 more obvious truth, calls a mid-rib, says : — " The flat stem of the Liverworts pre- 

 sents many varieties, consisting frequently of one simple layer of thin- walled 

 cells, or it exhibits in its axis the elements of the ordinary stem." This passage 

 exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, 

 to escape hypotheses they dislike. Schleiden, with the positiveness characteristic 

 of him, asserts the primordial distinction between axial organs and foliar organs. 

 In the higher Acrogens, he sees an undeniable stem. In the lower Acrogens, clearly 

 allied to them by their fructification, there is no structure having the remotest 

 resemblance to a stem. But to save his hypothesis, Schleiden calls that "a flat 

 stem," which is very obviously a structure in which stem and leaf are not differ- 

 entiated. He is the more to be blamed for this unphilosophical assumption, since 

 he is merciless in his strictures on the unphilosophical assumptions of other 

 botanists. 



Vol. II. * 



