THE GYMNOSPERMAE : CONIFERALES AND TAXALES 693 



Pines, persists until the seedling is 3 or 4 in. high, when the first foliar spurs 

 appear in the axils of the juvenile leaves. As the seedling continues to 

 develop the later formed juvenile leaves become progressively smaller and 

 pass over gradually into the scale leaves of the mature stem. This change 

 over to the spur leaves probably represents a recapitulation of a similar change 

 in the evolutionary history of the race (Fig. 695). 



Alternation of Generations 



We see then that in Pimis there is the same alternation of generations 

 that we have met with in previous types. The chief points of interest bemg 

 that in Pinus the gametophytes are still further reduced ; they are entirely 

 dependent upon the sporophyte for food and are retained within the spores. 

 In fact the male gametophyte is represented by little more than the essential 

 sex cells. In the female gametophyte an extensive tissue is formed, but its 

 function is to nourish the developing embryo, and it obtains the food to do 

 this, not by independent assimilation, but by transferring food from the 

 sporophyte for this purpose. It functions, then, merely to transfer food and 

 not as a supplier. 



There is a still further reduction in the number of megaspores, which is 

 now reduced to one, while there is a large production of microspores, only a 

 ver>' small proportion of which ever reach the female cone. Thus m Pinus 

 there is a great wastage of fertilizing material, and we shall find that in the 

 Angiosperms, methods have been found to reduce this wastage to a minimum. 



Male Cone 



Pinus Tree 



Megasporangium 

 Microsporangium 



Megaspore 



Microspore 

 (Pollen Grain) 



Embryo Sac 



Af)theridial Cell 

 Fig. 696.— Life-cycle ox Pi>ius syivestris. 



Nuclei 



