4i8 MORPHOLOGY OF GYMNOSPERMS 



and the Ginkgoales, and is a feature of some of the Taxaceae. Among 

 the Pinaceae the outer fleshy layer perhaps is represented in the 

 young integument, but does not develop, so that the stony layer is 

 the conspicuous superficial feature of the seed. The probability is 

 that the stony layer would not develop superficially in any event, 

 so that it would not be necessary to regard a layer or two of cells 

 overlying it as representing the outer fleshy layer. The development 

 of the outer fleshy layer among the Cycadales and Ginkgoales 

 is phylogenetically continuous from the Cycadofilicales and Cor- 

 daitales; but among the Taxaceae there is probably no such 

 continuity, but a reappearance of the activity of this layer in 

 certain genera. Among the Gnetales, and apparently in certain 

 Cordaitales, the single integument of the other gymnosperms is 

 replaced by two integuments, the inner fleshy layer having become 

 differentiated as a delicate inner integument, which appears 

 earlier than the heavier outer integument, which gives rise to 

 the outer fleshy and stony layers. In Gnetum the outer fleshy 

 layer develops the pulpy investment characteristic of the primitive 

 seeds; but in Ephedra this layer behaves as among the Pinaceae. 

 If any- general tendencies can be inferred from these facts in refer- 

 ence to the integument and testa, they are seen in the abortion of 

 the outer fleshy layer in the largest group of living gymnosperms, 

 and in the differentiation of the three-layered integument into two 

 integuments. 



The vascular supply of the ovule exhibits a very evident progressive 

 change. When the integument and nucellus become free only in 

 the region of the nucellar beak, the inner set of vascular strands is 

 shifted from the peripheral region of the nucellus to the inner fleshy 

 layer, and this situation persists among the cycadophytes. Curiously 

 enough, it reappears in Gnetum, but in that case it is associated with 

 the presence of the two integuments. Among Ginkgoales the outer 

 set of strands is suppressed; among Taxaceae the inner set is sup- 

 pressed; and among Pinaceae both sets have disappeared. The 

 general tendency, therefore, is to eliminate the vascular strands 

 from the ovule; but it is puzzling to find both sets absent from the 

 older Abietineae, and one set still present among the younger Taxineae. 



The presence of a pollen chamber is one of the most conspicuous 



