474 SCIENCE PROGRESS. 



from the middle meristematic group, and is thus of common 

 origin with the cortex. On this distinction Van Tieghem 

 (5 and 19) bases the statement that the root of most Mono- 

 cotyledons has no epidermis, while that of Dicotyledons 

 possesses a true epidermis. The strict application of 

 such a principle would surely lead to very inconsistent 

 results. 



Returning to plants with an apical cell, we may safely 

 conclude that, with the solitary exception of the origin of 

 the root cap, the first tangential cell-walls do not constantly 

 mark out the main histogenetic layers, from which the 

 principal system of tissues is derived. Such layers arise 

 in different ways in different cases, and at various distances 

 from the apex. 



As regards the Phanerogams, the researches of Van 

 Tieghem and Douliot (19) appear to show that in roots, 

 at any rate at their first origin, the histogenetic layers are 

 really distinct. We may be permitted to doubt whether 

 they always remain so throughout the entire develop- 

 ment. 



The question as to the apex of the stem in flowering 

 plants is really in a state of chaos, and we cannot claim 

 to have made any advance since the days of Sanio and 

 Hanstein. A paper published by Douliot in 1890 (20) 

 attempted to set the matter at rest. He adopted the 

 view that in Gymnosperms, as in most vascular Cryptogams, 

 there is a single apical cell, but his figures fail to support 

 this conclusion. Among the Angiosperms, Douliot found 

 in Dicotyledons generally the three initial layers required 

 by the theory of Hanstein, while in most Monocotyledons, 

 he reduced them to two, plerome and periblem having a 

 common initial group. 



Almost simultaneously with Douliot's paper a work 

 appeared by Ludwig Koch on the apex of the stem in 

 Gymnosperms (21). He decides against the existence of 

 an apical cell, and his figures, drawn from microtome sec- 

 tions, appear to be quite conclusive on this point. Just as 

 little, however, does he support the theory of histogenetic 

 layers. He finds, in agreement with previous observers, 



