Lecture XXIV. 209 



exhausted. In this way four embryos may be formed from the one 

 oosperm : only one of these, however, continues its development. 

 The others are absorbed along w r ith the material of the endosperm 

 by the successful one. In some other conifers the four cells 

 below the suspensors cling together and jointly give rise to one 

 embryo. 



The embryo now develops into a cylindrical mass of cells, lying 

 lengthwise in the ovule which is now becoming the seed. The 

 blunt end of this embryo is turned away from the micropyle and 

 becomes the growing region of its developing stem, and round it 

 grows a crown of embryonic leaves the cotyledons about twelve 

 in number. The other end of the embryo becomes the root, or 

 radicle, and is covered by a large root-cap. Round the embryo 

 and in close contact with it, is the gametophyte or endosperm, 

 which has by this time absorbed most of the nucellus. The only 

 remnant of this latter is a paper-like membrane intervening between 

 the hard seed-coat and the gametophyte. In this condition the 

 seed is shed from the seed-cone and by means of its wing is borne 

 on the winds often to considerable distances. 



After a period of quiescence the seed germinates if suitable 

 conditions prevail. The embryo then recommences its growth 

 and, absorbing the food material stored in the endosperm and 

 rupturing the seed-coat, pushes out its radicle through the micro- 

 pylar end of the seed. The stem below the cotyledons elongates 

 while the cotyledons remain in the seed and draw out the supplies 

 which it contains. Finally when all the stores are exhausted the 

 seed-coat is shoved off and the cotyledons expand, and the seed- 

 ling starts on its independent life. 



The short study we have devoted to the life-history and structure 

 of the Scots' Pine, insufficient though it be in many respects, 

 leaves us in no doutjj as to there being a general similarity between 

 the tree and the comparatively small ferns Aspidium and Selaginella. 

 The general manner in which they are constructed and the 

 peculiar metamorphoses which they undergo are fundamentally 

 similar in both cases. 



An alternation of generations has come to light in the life- 

 history of the pine. The fact that the gametophyte generation is 

 wholly enclosed in the sporophyte renders this less evident, but the 

 study of the origin and development of the pollen-grains and embryo- 

 sacs establishes beyond doubt that the former are homologous to 

 the microspores and the latter to the megaspores of Selaginella. 

 Thus the Pine is heterosporous. Further, the pollen-tube and its 

 contained cells are evidently homologous to the male gametophyte 



