3io 



BOTANY OF THE LIVING PLANT 



though as the cone grows older it becomes woody, and is by far the 

 more prominent feature of the two. Other interpretations of the 

 cone have been given ; but if this view be accepted, then the whole 

 cone is a simple flower bearing many complex sporophylls. Attached 

 to the upper face of the ovuliferous scale are two ovules, with their 

 wide micropyles directed downwards. Each ovule consists of a nucellus 

 corresponding to that of the Angiosperms, surrounded by a single 

 integument, and with a wide micropyle (m, Fig. 252). The pollen- 

 grains being produced in enormous numbers, and floating away on 

 the dry air of a June day, are scattered over the female cones, of 

 which the scales then stand apart to receive them. A drop of fluid 

 extruded from the micropyle catches them and is then absorbed. 



FIG. 252. 



Median section of ovule and scales in Pine at time of pollination (after Coulter). 

 6sc=bract scale, ov. sc= ovuliferous scale. n= nucellus. = integument. m= 

 micropyle. e.s. = embryo-sac. As the two ovules lie side by side, only one of them 

 is seen in the radial section, p pollen-grain or nucellus. 



Thus the pollen-grains are landed directly on the apex of the nucellus, 

 where they are constantly to be found in sections cut through the 

 ovule (p, Fig. 252). Excepting that there is no receptive stigma 

 the process is not unlike that in wind-pollinated Angiosperms. 



But differences of great comparative interest lie in the details 

 within the ovule itself. The ovule originates as in Angiosperms, 

 and as in them the embryo-sac is one cell of a tetrad produced in the 

 young nucellus ; its nucleus is therefore haploid. But an essential 

 difference is that in Pinus, and all other Gymnosperms, nuclear 

 division proceeds to a high number in the young embryo-sac. Cell- 

 formation follows, and the sac is thus filled before fertilisation by a bulky 

 tissue of the endosperm, or female prothallus (Fig. 253). At the time 

 of fertilisation, which in Pinus and Picea happens about the middle 

 of June of the year after pollination, the female prothallus bears at 

 its micropylar end three to six large archegonia, of which two commonly 

 appear in a median longitudinal section of it (Fig. 253, a). Each of 



