36 



MORPHOLOGY OF GYMNOSPERMS 



seem to be the units that make up the chambered integument 

 ("canopy") of Lagenostoma, and this has suggested to Oliver the 

 multiple origin of the integument. No stony layer is developed by 

 the testa, and the distribution of the vascular strands is practically 

 as in Lagenostoma, a single strand ascending deep in each rib and 



Figs. 35-39. — Physostoma elegans: fig. 35, diagrammatic median longitudinal 

 section; figs. 36-39, transverse sections at the levels A, B, C, D, respectively; b, vas- 

 cular bundles; t, "tapetum." — After Oliver (85). 



passing on into the corresponding tentacle. The pollen chamber 

 consists of the same circular crevice as in Lagenostoma, but the embryo 

 sac develops into the nucellar projection (fig. 35), so that the pollen 

 chamber really invests the upper end of the sac ("like the incurved 

 bottom of a wine bottle"). The peripheral region of the nucellus, 

 from the chalaza to the top of the embryo sac, is occupied by a remark- 

 able "secretory zone," made up of thin, oblong, tabular cells, and 

 representing what Oliver regards as the retention of a feature present 



