THE MORPHOLOGICAL COMPOSITION OF PLANTS. 83 



cotyledons, but not always (Dioscoracece have lateral cotyl- 

 edons), but the plumule may push through it (Grasses) or 

 make its exit sideways (Palms), or be formed at the side 

 (Alisma) ; and Dicotyledons very similarly. 



" The occurrence of completely sheathing leaves in grasses 

 is perhaps correlated with the absence of cambium, but 

 grasses are an aberrant type among monocotyledons, and 

 secondary thickening is only found in very few genera of 

 this class, so that the correlation is, so to speak, negative and 

 indirect It is clear that the greater part of the dis- 

 cussion will have to be re-written." 



For the reasons assigned in the preface I cannot undertake 

 to re-write the discussion, as suggested. It must stand for 

 what it is worth. All I can do is here to include along with 

 it the foregoing criticisms. 



I may, however, indicate the line of defence I should take 

 were I to go again into the matter. The objections are based 

 on the structure of existing Liverworts and Phaenogams. But 

 I have already referred to the probability — or, indeed, the 

 certainty — that in conformity with the general principle set 

 forth in the note to Chapter I, we must conclude that the 

 early types of Liverworts out of which the Phagnogams are 

 supposed to have evolved, as well as the early types of 

 Phgenogams in which the stages of evolution were presented, 

 no longer exist. We must infer that forms simpler than 

 any now known, and more intermediate in their traits, were 

 the forms concerned; and if so, it may be held that the 

 incongruities with the hypothesis which are presented by 

 existing forms, do not negative it. The scepticism my critic 

 himself expresses respecting the current interpretation is a 

 partial justification of this view. Moreover, his admission 

 that the theory set forth " fits in well with the phenomena 

 exhibited by phanerogamic shoots," must, I think, be regarded 

 as weighty evidence. On the Evolution-hypothesis we are 

 obliged to suppose that the Monocotyledons and Dicotyledons 

 respectively arose by integration of fronds; and if to the 



