PHANEEOGAMIA 173 



the single spore. It was formerly described as the embryo sac, 

 as the embryo ultimately makes its appearance in it. During 

 its growth, which is very considerable, it absorbs the cells aromid 

 it, often to such an extent that it leaves nothing of the original 

 nucellar tissue. It may even encroach upon the integuments. 

 It then forms a large cell, clothed only by the integuments of 

 the sporangium. If the absorption of the nucellus is not 

 complete, the remainder of its cells form the tissue already 

 alluded to as the perisperm. 



The nucellus of the ovule is seen thus from its development 

 to be the macrosporangium of the Phanerogam, and to corre- 

 spond to the macrosporangium of Selaginella. It differs from 

 the latter in that the mother cell of the spore becomes itself the 

 spore, whereas in Selaginella the corresponding cell divides 

 twice, forming four macrospores. It differs further in not 

 secreting a thickened coat, its wall always remaining thin and 

 delicate, except in the Cycadese, where it is double. 



The integuments of the ovule correspond to the indusium, 

 which in Salvinia and Azolla has been shown to be two-layered 

 and to grow over and enclose the sporangia. The integuments 

 do not completely close over the ovule, while in the former case 

 they shut the sporangia in entirely. 



The structure in the Phanerogams recalls the condition in 

 Azolla more than in Salvinia. In Azolla the sorus of macro- 

 sporangia consists only of one, and this one is invested by the 

 indusium just as the o\n^de is surrounded by its integuments. 

 The macrosporangia in the Phanerogams are thus solitary as in 

 Azolla, or in other words the sorus is monosporangiate. 



Though this is the usual course of development in the 

 Phanerogams, there are many variations of the process known. 

 More than one row of cells in the nucellus maj^ give rise to 

 archesporia, and consequently more than one embryo. sac or 

 macrospore may be produced. Eventually only the central one 

 becomes mature, the others perishing early. In some cases, 

 instead of a single spore mother cell occurring, this divides into 

 a number of sporogenous ceUs, of which again several may begin 

 to develop into macrospores, though ultimately only one of 

 them matures. In a few plants the original archesporium does 

 not cut off any tapetal cells. 



The features of the gametophyte generation and the embryo- 

 geny of the sporophyte differ considerably in the Gymnosperiiis 

 and the Angiosperms, and will be best described in connection 

 with those respective groups. 



