ORGANIC EVOLUTION 



149 



Reproduction in the pine. In the pine we again meet with 

 two kinds of spores, microspores and macrospores. With 

 the microspores of the seed plants we have already 



become acquainted 

 under their older 

 name of pollen 

 grains, and wi^h 

 the microsporan- 

 gia, as the pollen 

 sacs of the anth- 

 ers. The latter are 

 developed in pairs 

 in the lateral mar- 

 gins of scales of 

 the well known 

 little cones, which 

 shower down yel- 

 low pollen in early 

 spring. Such cones are remotely comparable to the fruit- 

 ing spikes of Selaginella and Equisetum. 



The microspore or pollen grain develops into a male 

 prothallium or gametophyte of extremely small size, con- 

 sisting when grown of only a few cells, one of which on 

 division gives rise to two sperm nuclei. These lack loco- 

 motor organs and do not swim abroad when mature ; instead, 

 the microspore (or the prothallium developed from it) is 

 transported bodily by the wind to the vicinity of the egg 

 cells. 



The macrospores (fig. 95) are developed in similar fruiting 

 spikes, which when mature are the familiar cones of the 

 pines (and other conifers). There are but two sporangia 

 upon the base of each scale and in these the macrospores 

 are developed singly. As in selaginella they are of large 

 size, owing to the food stored in them. They differ from 



FIG. 94. Chickweed. a, flower; b, fruit. 



