150 GENERAL BIOLOGY 



those of selaginella most markedly in not being discharged 

 at maturity. They remain permanently inclosed in the 

 macrosporangium (usually known in botany by its older 

 name nucellus), and invested closely by a thin layer of 

 integument, the whole structure being known in botany as 

 the ovule. 



The investing integument does not close completely over 

 the macrosporangium, but leaves a little hole at one end, 

 the micropyle. Hither the microspore is brought by the wind 



FIG. 95. Diagrams of spore development in the pine, a, longitudinal sec- 

 tion of the staminate small cone b, one of the scales from the same, 

 showing at c, the pollen cavities (microsporangia) ; c, the single pollen 

 grain, divided into two cells, and bearing a pair of thin flat winged 

 processes at its sides; d, a scale from the pistillate cone, bearing a pair of 

 macrosporangia: (o) ovules; e, a single ovule, in section, with a pollen 

 grain lying inside' the pollen cavity at the top: m, macrospore; f, macro- 

 sporangmm penetrated by two pollen tubes (t, f) ; two sperm nuclei (n) 

 shown in one of them; the macrospore (m) is developed into the female 

 prothallium, bearing archegonia (a, a) each containing an egg. 



previous to fertilization. The macrospore remains 

 thus in captivity. Within it develops a mass of cells 

 which is the female prothallium, in the apex of which 

 adjacent to the micropyle several reduced archegonia are 

 developed, and in each of these an egg nucleus is produced. 

 The microspore, previously lodged at the micropyle, 

 develops a rhizoid-like process (the pollen tube) from the 

 antheridial cell, and this penetrates to the egg and liberates 



