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 

 proceeds 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 Selaginella ; this 

 fact has its bearing on the origin of their differentiated state. 



The sporangium of /. laacstris 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 /. lacustris, 

 though apparently the number may be smaller in 7. 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 7. alpinum ; and 

 this is completely in accordance with the radially extended form of the 

 mature sporangium of Isoetes. Moreover, the differences beween Wilson 

 Smith's description for 7. echinospora and my own for 7 lacustris suggest 

 that differences of radial extension of the sporangium exist in different 

 species of Isoetes 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 /. echinospora 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 a 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, Hot. 

 Gaz., 1900, p. 225). 



