LIFE HISTORY OF PINUS 89 



density of their C3'toplasm^ which is almost identical with that of 

 the prothallium, while the cells of the nucellus are scantily 

 supplied with cytoplasm. These cells divide karyokinetically, 

 and, as they increase in number, they press against the adjacent 

 cells of the nucellus which become flattened against this con- 

 stantly advancing tissue, and are absorbed, only to give place 

 to other cells which meet a similar fate. Sometimes absorption 

 seems to precede the outward march of the spongy tissue, so 

 that this tissue is separated from the normal nucellus by a clear 

 space made up of cells of the nucellus which have lost all their 

 protoplasmic content, but which have not as yet suffered collapse 

 (figs. 157, 158, plate XV). The parietal layer of cytoplasm 

 which constitutes the endosperm remains always in closest con- 

 tact with the inner surface of this tissue (fig. 71, plate VI). 



The cells of the spongy tissue are still prominent when the 

 endosperm becomes a solid multicellular body. Soon after- 

 wards, however, they show signs of disintegration, and at the 

 time of fertilization they have, as a rule, entirely disappeared 

 as cells, only the remnants of the cell-walls remaining. The 

 spongy tissue is then represented by a deeply staining fibrous 

 body of no definite structure which persists between the 

 gametophyte and the nucellus (figs. 162, plate XV, and 72, 

 plate VI; 73, plate VII). 



The Nature and Function of the Spongy Tissue. — The 

 prominent character of the cells surrounding the prothallium in 

 certain Gymnosperms has been commented upon, in a general 

 way, by all students of the Ahieti^iece ; but, as was noted by the 

 writer in 1900 and 1901 and confirmed by Coker in Taxodium^ 

 1902, the true nature and function of these cells seem to have 

 escaped entirely the notice of previous writers, as they have in- 

 variably been described as tissue showing evidence of breaking 

 down. After a preliminary note regarding the nature of this 

 tissue was sent to press in 1900, Lang ('00) described a similar 

 layer of cells about the endosperm in Stangeria. He designated 

 them as sporogenous cells and " possibly tapetal in nature." 



As recently stated (1903),^ these cells may possibly represent 

 sporogenous tissue, each cell being a potential macrospore- 



1 See note at end of Appendix. 



Proc. Wash. Acad. Sci., August, 1901. 



