50 MORPHOLOGICAL DEVELOPMENT. 



ready to entertain the hypothesis that during the evolution 

 of the phaenogamic type, the distinction between leaves and 

 axes has arisen by degrees. 



With our preconceptions loosened by such facts, and 

 carrying with us the general idea which such facts suggest, 

 let us now consider in what way the typical structure of a 

 flowering plant may be interpreted. 



§ 192. To proceed methodically, we must seek a clue to 

 the structures of Phanerogams, in the structures of those 

 inferior plants that approach to them — Archegoniatce. The 

 various divisions of this class present, along with sundry 

 characters which ally them with Thallophytes, other charac- 

 ters by which the phsenogamic structure is shadowed forth. 

 While some of the inferior Hepaticce or Liverworts, severally 

 consist of little more than a thallus-like frond, among the 

 higher members of this group, and still more among the 

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

 Archegoniates (or rather Ehizoids) have foliar expansions 

 that are indefinite in their forms; and some have quite 

 definitely-shaped leaves. Roots 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 formed of only a single layer of cells; and 

 there a double layer gives them a higher character — a differ- 



* Schleiden, 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 Liver- 

 worts presents 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 wh'ch 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 Archegoniates he sees an 

 undeniable stem. In the lower Archegoniates, 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 obviously a structure in which stem and leaf are not differ- 

 entiated, lie is the more to be blamed for this unphilosophical assumption, 

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

 other botanists. 



