DISCUSSION OF RESULTS. 43 



STELAR MORPHOLOGY. 



Some views already published (1906) on this point may be repeated 

 here. The development of the seedling- stem supports the idea of Jeffrey 

 and Boodle of the phytogeny of the fern-stem. We first have the pro- 

 tostele, then the ectoploic siphonostele, and finally the solenostele. But 

 there is no evidence of any influence of the tissues outside the vascular 

 tiibe upon those inside. Each interior tissue is established before it comes 

 into communication with its external homologue. 



HOMOLOGY OF TISSUES. 



Indeed, homology of tissues can not be determined either by continuity 

 or by origin in the meristem. We may not homologize the medulla of 

 Dennstcedtia with the central xylem of Lygodium simply because both 

 arise from the inner ends of the segments of the stem-initial. Much less 

 could we identify the inner endodermis of Dcnnstirdtia with any of the 

 xylem of Lygodium. On the other hand, the continuous endodermal 

 layers of root, stem, and leaf in Dennstfedtia must be considered as one 

 homog-eneous tissue. But in the root-tip the endodermis arises just out- 

 side and the pericycle just inside the second periclinal wall in each seg- 

 ment. In the stem the endodermis is the result of the last (second) peri- 

 clinal division in a layer of plerome which also gives rise to the pericycle. 

 The same is true of the leaf an organ which grows at first by a three- 

 sided initial, then by a two-sided, and finally by a group of marginal cells. 

 In Dennstfedtia pioi dilobula, therefore, tissues are homologous which have 

 the same structure and function, in spite of their differences of origin 

 (</". Goebel, 1900). 



The same conclusion is indicated by the long-familiar fact that in roots 

 of dicotyledons the undoubtedly homologous primary tissues arise from 

 at least five different types of root -tip (De Bary, 1884, p. 12). Indeed, 

 radically different types of tip may occur in allied genera, as in Nymphcra 

 and Nuphar. And it is not impossible that different types may be found 

 in different roots of the same individual plant. The recent discussion of 

 stem -tips raises the same point in questioning 1 the validity of Hanstein's 

 tissue layers (Schoute, 1903, etc.). It seems reasonably certain that Han- 

 stein's layers are not of very wide application. In the face of so much 

 evidence, also, Van Tieghem's denial of an epidermis to the roots of ferns, 

 monocotyls, and Nymphaeaceae becomes quite valueless. These plants 

 have just as real an epidermis as have others. 



INDEPENDENCE OF MERISTEM AND MATURE TISSUES. 



The fact is that the development and structure of the mature tissues 

 are to a large extent independent of the development and structure of 

 the meristem from which they are derived. The lowest plant with cell- 

 division in three planes is essentially meristematic. From such a begin- 



