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 phylogeny 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 
tube upon those inside. Each interior tissue is established before it comes 
into communication with its external homologue. 
HOMOLOGY OF TISSUES. 
Indeed, homolog’y of tissues can not be determined either by continuity 
or by origin in the meristem. We may not homologize the medulla of 
Dennstedtia 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 Dennstedtia with any of the 
xylem of Lygodium. On the other hand, the continuous endodermal 
layers of root, stem, and leaf in Dennstedtia must be considered as one 
homogeneous 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 Dennstedtia punctilobula, therefore, tissues are homologous which have 
the same structure and function, in spite of their differences of origin 
(of. 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 Mymphea 
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 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 Nymphzeaceze 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- 
