GNETALES S^I 



Study of the ovule and seed in comparison with those of other gymno- 

 sperms. The outermost envelope is undoubtedly the so-called perianth 

 of Gnetales, very closely investing the ovule, with its own vascular 

 supply, and appearing before the integuments. This last fact gave 

 rise to the impression that the three envelopes appear in acropetal 

 succession. The seed becomes slightly stalked in this perianth, and 

 the two integuments appear in basipetal succession. The inner one 

 extends above to form the long micropylar tube, and at the maturity 

 of the seed it completely invests the nucellus (at this time replaced by 

 the endosperm) as a papery layer. The outer integument becomes 

 differentiated into an outer fleshy layer and an irmer stony layer, the 

 latter completely investing the seed, the former being chiefly developed 

 in the region of the nucellar beak. Two sets of vascular strands are 

 present, the outer set traversing the fleshy layer of the outer integument, 

 the inner set traversing the inner integument. 



In Cycadophytes, Ginkgoales, and Coniferales a single integu- 

 ment becomes differentiated into a testa of three layers: outer fleshy, 

 stony, and irmer fleshy. In Gnetum the same three layers are present, 

 but the inner fleshy one becomes differentiated in ontogeny as a 

 separate integument. In all cases this innermost layer finally forms 

 a papery lining of the stony layer. Among the Pinaceae the outer 

 fleshy layer is represented in the integument, but it does not develop 

 into the pulpy investment that characterizes the Cycadales, Gink- 

 goales, and Taxaceae. The variation in the distribution of the 

 vascular strands among these layers should be summarized. Among 

 the more primitive Cycadofilicales and Cordaitales, in which the 

 nucellus is relatively free from the integument, the outer set of strands 

 traverses the outer fleshy layer (or integument) and the inner set 

 traverses the peripheral tissue of the nucellus. In other Cycadofili- 

 cales and Cordaitales, however, and in Cycadales, in which the nucel- 

 lus and integument are free only in the region of the nucellar beak, 

 the inner set of vascular strands traverses the inner fleshy layer; and 

 this is the condition of Gnetum, except that this layer has become 

 differentiated as an inner integument. In Ginkgoales the outer set 

 of strands (belonging to the outer fleshy layer) is suppressed; in 

 Taxaceae the inner set (belonging to the inner fleshy layer) is sup- 

 pressed; and in Pinaceae both are suppressed. 



