SPORE-PRODUCING MEMBERS 319 
the trabeculae have a common origin with the fertile sporogenous cells: 
_ there has in fact been a sterilisation of potentially fertile tissue, which 
praceeds to a greater length in the megasporangium than in the micro- 
sporangium. The early development of both types of sporangia is alike 
up to a fairly advanced condition, as is the case also in Se/aginedla; this 
fact has its bearing on the origin of their differentiated state. 
The sporangium of JZ. /acustris originates from superficial cells of the 
leaf-base of small number, lying below the ligule (Fig. 165 a.) The cell seen 
immediately below the ligule in the longitudinal section of the young 
leaf forms the velum: the rest show some evidence of common origin 
by earlier anticlinal segmentation: this may very well have been so, but 
the comparative interest begins with their periclinal divisions, and it is 
then that a basis appears for comparison with what has been seen in 
Lycopodium. The periclinal division appears first in the central part of 
the young sporangium, and thence it extends in either direction: in the 
longitudinal section some four-or five cells are involved in JZ. lacustris, 
though apparently the number may be smaller in /. echinospora.1 Com- 
paring this with the condition as seen in Lycopodium, it appears to be 
an advance on even the most complex type, such as Z. alpinum; and 
this is completely in accordance with the radially extended form of the 
mature sporangium of J/svefes. Moreover, the differences beween Wilson 
Smith’s description for Z. echinospora and my own for J. dacustris suggest 
that differences of radial extension of the sporangium exist in different 
species of Jsve¢es similar to those which have been shown to occur within 
the genus Lycopodium. But there does not appear to be any such cor- 
relation of them with the morphological differentiation of the plant at 
large as that which was traced in Lycopodium, and gave a special interest 
to the sporangial differences in that genus. 
The internal cells thus cut off by the first periclinal divisions are 
destined to be sporogenous; but the first periclinal divisons thus initiated 
do not absolutely define the future sporogenous tissue: it has been 
repeatedly seen that additions to it may be made by subsequent periclinal 
division of the superficial cells, especially in the middle region of the 
1 Wilson Smith found in 7. echixospora that he was able to trace the origin of the 
sporangium back in longitudinal sections of the leaf to a single cell lying between the 
ligule and the leaf-base: this corresponded to a transverse row of three to five cells, which 
formed the rudiment of the sporangium ; but the cell thus recognised in the longitudinal 
section also formed the velum, which on that account he accepts as a sterilised part of 
the sporangium. Doubtless this is a logical outcome of a last analysis of cell-origins, 
provided it be assumed that all things are homologous which have a common origin from 
ultimate parent cells (see Chapter VIII.). But is there any other line of evidence than 
that of cell-origin to show that the velum was ever « part of a sporangium, or anything 
but sterile? Without such evidence the mere fact of common origin from a very early 
segmentation seems a somewhat shadowy ground for the conclusion which Wilson Smith 
proposes. If this criterion of homology be accepted, then all parts of the plant are 
ultimately homologous, for they all originate from the ovum. (See Wilson Smith, Bot. 
Gazj, 1900, p. 225). 
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