208 



Practical Plant Biology. 



In the spring and early summer the megaspore, as we have 

 seen, germinates within the nucellus and becomes filled with the 

 tissue of the gametophyte (prothallus, endosperm) and forms its 

 archegonia. During the germination of the megaspore the pollen 

 recommences its development. The tube-cell penetrates more 

 deeply into the nucellus and the body-cell breaks away from the 

 stalk-cell. Its cell-wall disappears and it divides to form two cells 

 which pass into the hypha-like tube-cell growing through the 

 nucellus towards the archegonia in the gametophyte. These two 



cells, as their subsequent history shows, 

 are the sperms or male gametes. 

 They are conveyed in the pollen-tube 

 to the neck of one of the archegonia 

 and when the tube forces its way be- 

 tween the neck-cells, its tip becomes 

 dissolved and the two sperms along 

 with the other nuclei of the pollen- 

 tube pass into the ovum. The nucleus 

 of one of the sperms fuses with the 

 nucleus of the ovum and fertilisation 

 is effected. 



Evidently we should regard the 

 filiform growth from the pollen-grain 

 as a male gametophyte which develops 

 parasitically on the nucellus, and when 

 mature produces the sperm-cell which 

 fertilises the ovum. Thus the two 

 gametophytes of the pine are very 

 diminutive in size and parasitic in 

 their nutrition. They both reach 

 FIG. 68. Pinus silvestris, maturity one year after the germina- 

 ovule at time of fertilisation, , f li 



longitudinal section, x 18. ti on of the spore 



e, endosperm ; i, integument ; I he nucleus of the OOSperm divides 

 w, micropyle; , nucellus; immediately into two and then into 

 o, ova;/, pollen-plant. foun These four nude j move tQ 



the basal end of the oosperm and 



become shut off from one another by cell-walls. The four cells 

 so formed each gives rise to a filament of four cells. The cell 

 of each filament furthest from the neck of the archegonium becomes 

 an embryo and the remaining part of the filament elongates rapidly 

 and is called the suspensor. It has the function of pushing the 

 growing embryo more deeply into the surrounding gametophyte 

 (endosperm) according as the food materials in that tissue become 



