4i6 GYMNOSPERMS 



epidermis, becoming hard, is the outer protecting layer. This con- 

 dition is universal in Pteridophytes and in the Carboniferous gym- 

 nosperms. 



The ovulate sirobiliis. — ^The ovulate strobilus has received more 

 attention than the staminate, doubtless because it can be collected 

 during several months and is quite conspicuous for some time before 

 the seeds ripen and fall off (fig. 389). The figure shows some ovulate 

 strobili of the plant of Gnctum gnemon (fig. 382) grown at the Uni- 

 versity of Chicago. At this time the plant was about 20 years old 

 and this was the first and only time it ever produced strobili. The 

 characteristic leaves, bearing such a close resemblance to dicotyls, 

 are also well shown in this figure. 



The axis of the ovulate strobilus in Gnetum gnemon has a pair of 

 opposite sheathing bracts at the base, followed by five or six whorls 

 of ovules, with five to seven ovules in a whorl (fig. 390). Sometimes 

 there is a terminal ovule. Although there are so many ovules, most 

 of which reach the pollination stage, and even the fertilization stage, 

 only a few, perhaps from two to five, complete the entire develop- 

 ment and become capable of germination. 



In the development of the strobilus the pairs of concrescent bracts, 

 usually about seven, forming a series of cups, are first to appear, 

 followed by the nucelli of the ovules in a ring in the axils of the 

 bracts (fig. 391). 



Ovules in ovulate strobili have two integuments and a perianth, 

 while those in the staminate strobili lack the outer integument. In 

 the latter case, a rudiment of the outer integument appears, but 

 soon aborts. In normal ovules the order of appearance is perianth, 

 outer integument, inner integument. As in the other two genera, the 

 inner integument becomes prolonged into the characteristic micro- 

 pylar tube. In the mature seed it becomes reduced to a thin, papery 

 layer. The outer integument finally becomes differentiated into an 

 outer fleshy layer and an inner stony layer, so that the seed has three 

 seed coats, as in the cycads, a stony layer in the middle, with a 

 fleshy layer outside, and a thin, dry, membranous layer inside. 

 These three layers are very common in gymnosperms, and have led 

 some into curious interpretations. The three layers appear in typical 

 form in the cycads, where it is said that the integument is diflerenti- 



