222 Nichols, A morphological study of Juniperus communis var. depressv. 



that in Pinus the cells never reach the center "without having 

 first divided by cell walls", and that "a ring of tissue composed 

 of longer or shorter cells is formed rather early in the inward 

 growth of the prothallium". This appears to he precisely what 

 happens in the form under discussion, as may be seen from fig. 77. 

 The further development of the prothallium was not followed in 

 detail. Fig. 3, however, shows its appearance shortly after the 

 cells have become closed in, and fig. 4 represents a similar section 

 at about the time of fertilization. 



The phylogenetic importance of the megaspore membrane in 

 Gymnosperms has been recently emphasized by Thomson (1905 a) 

 who states that "the megaspore coat closely resembles that of a 

 microspore both in its structure and its chemical composition, and 



Fig. 3. Fig. 4. 



Radial section through lower portion Section similar to fig. 3 at about the 

 of prothallium shortly after formation time of fertilization. 



of cross walls. X 190- X l9 °- 



thus affords additional evidence of the free-sporing nature of the 

 ancestral forms of the Gymnosperms". He finds that this structure is 

 present in all the groups and sub-groups of these seed plants, except 

 the Taxeae, from the ovule of whose forms it is entirely or almost en- 

 tirely eliminated. Thomson describes the megaspore membranein J. 

 sabina and J. virginiana, while Noren (1907) describes that of 

 J. communis, and the observations of the writer are in accord 

 with those of these two authors. The coat is formed during the 

 free nuclear period of the embryo sac and reaches its highest 

 development at about the time of fertilization. It is 2.5 — 3 ß 

 thick in the basal region of the prothallium and of fairly uniform 

 thickness throughout, except at the micropylar end of the embryo 

 sac where in the archegonial region it becomes quite thin. In 

 section (fig. 73) two distinct layers are seen, the outermost of 



