CONIFERAE: THE S< OTS PINE 



projects and grows down (Fig. 420, II. The hypocotyl elon 

 and the seed is carried above ground. The tips of the cotyl 



remain within it till the store is exhausted, when it is c tnd 



the cotyledons expand round the central plumule (Fig. 420, III.). 

 Thus the seedling is established. 



Comparison between Pteridophytes and Seed-Plants. 



The Gymnosperms offer natural lines of comparison on the one 

 hand with the heterosporous Pteridophyta, and on the other with the 

 Angiosperms. The general features of the life-cycle are the same in 

 them all. Hence it may be concluded that the comparisons are 

 legitimate, as between cognate organisms. In all of them the sporo- 

 phyte is the dominant " plant," while the gametophyte shows pro- 

 gressive degrees of elimination as an independent vegetative structure. 

 Comparing Pinus as an average Gymnosperm with Selaginella, which 

 may be taken as a fair example of a heterosporous Pteridophyte, the 

 " plant " bears in either case two kinds of sporangia. The micro- 

 sporangium of Selaginella corresponds to the pollen-sac of the Pine, 

 since both produce spore-mother-cells, and these after tetrad-division 

 form microspores, which are shed on rupture of the sporangial wall. 

 The megasporangium of Selaginella corresponds to the nucellus of the 

 ovule of Pinus, which is, however, protected by the extra covering 

 of the integument. In the ovule, as in the megasporangium of Sela- 

 ginella, a tetrad-division leads up to the formation of the megaspore. 

 In Pinus only one megaspore or embryo-sac is matured. In 

 Selaginella commonly four, and occasionally more : but sometimes 

 two, or even one only. Thus in point of origin, in the manner of 

 production, and sometimes also in number, the megaspore of Sela- 

 ginella corresponds to the embryo-sac of Pinus. An essential 

 difference may appear to lie in the fact that in Selaginella the mega- 

 spore with its thick protective wall is shed on rupture of the 

 megasporangium : while in Pinus the megaspore or embryo-sac is 

 retained within the nucellus, and is thin-walled. But this retention 

 of the megaspore within the sporangium gives the biological advantage 

 of continued nutrition, to which a thick and rugged wall would be an 

 unnecessary obstacle. The facts thus point to the conclusion that 

 the thin-walled embryo-sac of Pinus is a retained megaspore, and that 

 we see in it a derivative state which has been universally adopted by 

 Seed-Plants. (Compare Fig. 412, IV., with Fig. 423). ]t has already 

 been noted that certain fossil Lycopods had done the like. 



