542 BOTANY 



PART II 



TUBE, and an ANTHERIDIAL MOTHER CELL which ultimately gives rise 

 to two GENERATIVE CELLS. The pollen-tube, the wall of which is 

 continuous with the intine of the pollen grain, ruptures the exine 

 and penetrates, owing to its chemotropic irritability, into the tissue 

 of the macrosporangium (cf. p. 352). The antheridial mother cell 

 passes into the pollen-tube and sooner or later gives rise to two 

 generative cells which reach the embryo sac and egg-cell by passing 

 along the pollen-tube. The name Siphonogams has been applied to 

 the seed -plants on account of the common character of the group 

 afforded by the formation of a pollen-tube. 



The results reached by the above survey may be summarised by 

 saying that the Phanerogams continue the series of the Archegoniatae 

 and agree with the latter in exhibiting an alternation 'of generations 

 (cf. the Scheme on p. 543). While the asexual generation becomes 

 more complex in form and more highly organised, there is a 

 corresponding reduction of the sexual generation. The female sexual 

 generation is enclosed, throughout its whole development, in the 

 asexual plant, and only becomes separated from the latter in the seed, 

 which further contains as the embryo the commencement of the 

 succeeding asexual generation. 



The investigations made of recent years into the phenomena of the 

 reduction division (cf. p. 204) in the spore-mother-cells of Archegoniates 

 and Spermatophyta have resulted in a confirmation of the limits of the 

 two generations in the latter ( 2 ). The number of chromosomes char- 

 acteristic of any plant is diminished to one-half, during the divisions 

 that lead to the origin of the sexual generation, and the full number 

 of chromosomes is not again attained until fertilisation takes place. 

 The asexual generation has always the double number, the sexual 

 generation the single number of chromosomes. The gametophyte is 

 haploid, the sporophyte diploid. 



The Spermatophyta are divided into two classes which differ in 

 their whole construction: (1) the Gymnosperms, with naked seeds; 

 (2) the Angiosperms, with seeds enclosed in an ovary. 



The names of these classes indicate the nature of one of the most 

 important differences between them. THE CARPELS OF THE ANGIO- 

 SPERMS FORM A CLOSED CAVITY, THE OVARY, WITHIN WHICH THE 

 OVULES DEVELOP. SUCH AN OVARY IS WANTING IN THE GYMNO- 

 SPERMS, THE OVULES OF WHICH ARE BORNE FREELY EXPOSED ON 

 THE MACROSPOROPHYLLS OR CARPELS. 



The Gymnosperms are the phylogenetically older group. Their 

 construction is simpler and in the relations of their sexual generation 

 they connect directly with the heterosporous Archegoniatae ; they 

 might indeed be treated as belonging to this group. 



The Angiosperms exhibit a much wider range in their morpho- 

 logical and anatomical structure. The course of their life-history 

 differs considerably from that of the Gymnosperms, and without the 



