332 



GENEBAL BOTANY 



by the microspores' falling into the space between the open 

 megasporophylls and the axis of the strobilus. After pollination 

 the megasporophylls close by excessive growth on the abaxial 

 surfaces, and the cones gradually change their position, owing 

 to carpotropic movements, finally assuming the pendulous posi- 

 tion shown in Fig. 196. These ovulate strobili of the spruce are 



more complex in structure than the 

 staminate strobili, since each ovulate 

 strobilus bears on its axis two kinds 

 of scales, or modified leaf structures, 

 instead of one, as in the staminate 

 strobili. The large scales which con- 

 stitute the conspicuous part of the 

 mature cone, or strobilus, are the 

 ovuliferous scales, each of which bears 

 two megasporangia, or ovules, at its 

 base. These large ovuliferous scales 

 really arise as adaxial outgrowths 

 from very small scales which are only 

 evident in the early stages of the 

 strobilus, before the ovuliferous scales 

 have outstripped them in growth 

 (Fig. 197, c). The large ovuliferous 



FIG. 196. Spruce cones in 



June after pollination 

 These cones were photographed scales probably represent two sporo- 

 about a month later than those phylls of a reproductive branch, which 



represented in Fig. 195. Note J . r . 



the change in size and position of grew in the axils of leaves correspond- 

 the cones at the time of poiiina- 



tion and during seed formation 



m g to t h e minute scales of the young 

 cones. For our purposes we may prop- 

 erly term the ovuliferous scales megasporophylls, and consider 

 the ovulate strobilus a compound strobilus with both bracts 

 and sporophylls, instead of a simple strobilus, like that of the 

 staminate cones. 



The megasporangia, or ovules, of the spruce are similar to 

 those of the cycad, with a single integument surrounding the 

 sporangium tissue proper. Each megasporangium produces a 

 single large megaspore (Fig. 197, 6), which finally occupies 

 a considerable portion of the sporangium. The history of its 



