504 



BOTANY 



I'AllT II 



MC— 



tive cells or the male sexual cells. These are of very different form in the various 

 orders of Gymuospernis. The simpler development in the Coniferae, which was 

 known earlier, may first be followed. 



The Abietineae have two generative nuclei of unequal size in the one proto- 

 plasmic body ; only the larger nucleus which goes first is fertile (Fig. 459 E). 

 2'axus has only one generative cell, its sister-cell being greatly reduced. Two equal 

 generative cells are normally present in the Cuj^ressineae. In Araucaria numerous 

 nuclei develop in the pollen-tube ; this case requires more accurate investigation. 



As a rule the macrosporophylls 

 bear two macrosporangia ; these 

 consist of a nucellus and usually a 

 siugle integument. The nucellus 

 often becomes of large size by the 

 numerous periclinal divisions which 

 take place in the more superficial 

 layers of cells. In the same way 

 the sporogenous tissue, which orig- 

 inates immediately below the epi- 

 dermis, becomes deeply placed by 

 the separation of numerous layers 

 of cells (Fig. 460). Notwithstand- 

 ing the considerable size of the 

 sporogenous tissue only one cen- 

 trally placed macrosijore-mother- 

 cell is as a rule developed, all the 

 other cells remaining sterile. The 

 mother-cell undergoes a tetrad 

 division (Fig. 461), and of the four 

 resulting cells only one develops 

 into an embryo-sac (macrospore). 

 This, as it increases in size, first 

 crushes its sister-cells and later the 

 whole sporogenous complex of cells. 

 Mean while, .by the repeated division 

 of the nucleus and protoplasm, the 

 macrospore becomes filled with the 

 tissue of the ]irothallium (Fig. 462). 

 The archegonia are formed at the 

 apex of the prothallium ; each 

 consists of a large ovum and a short 

 neck. As in the Pteridojjhytes a 

 small ventral canal-cell is cut otf from the egg-cell shortly before fertilisation. 

 Fertilisation takes place by the entrance into the ovum of one of the generative cells, 

 tlie development of which has already been followed ; this enters from ithe pollen- 

 tube, which penetrates between the cells of the archegonial neck. The nucleus of 

 fertilised ovum results from the fusion of the male and female sexual nuclei (Fig. 

 445), and the jirotoplasm is in jiart derived from the male cell. The development 

 of the embryo from the fertilised ovum presents great dill'creuccs in the several 

 orders and even genera, and the following description applies to the species of 

 I'inus. (Fig. 46-3.) 



By two successive divisions of the nucleus four nuclei are formed which pass to 



Fio. 462. — Median longitudinal section of an ovule of 

 Picen excelsa. e. Embryo-sac filled with the pro- 

 tlialliuin ; a, archegoniuni showing ventral (o) and 

 neck portion (c) ; «, nucleus of egg-cell ; 7io, nucellus ; 

 J), polhm-grains ; /, pollen-tube ; i, integument ; s, 

 seed-wing, (x 0.) 



