476 



BOTANY 



constituent parts are always distinguishable in the germinating pollen- 

 grain ; these are a VEGETATIVK CELL which grows out as the POLLEN- 

 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. 286). 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 



Fio. 439. Hcmerocallis fulva. A, Transverse section of an almost ripe anther, showing the locnli 

 ruptured in cutting ; p, partition wall between the loculi ; a, groove in connective ; /, vascular 

 bundle ( x 14) ; B, transverse section of young anther ( x 28) ; C, part of transverse section of a 

 pollen-sac; pm, pollen -mother- cells ; t, tapetal layer, later undergoing dissolution ; e inter- 

 mediate parietal layer, becoming ultimately compressed and disorganised ; /, parietal layer of 

 eventually fibrous cells; e epidermis (x 240); D and E, pollen-mother-cells after division 

 (x 240). 



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. 

 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 exhaustive 

 investigations made of recent years into the phenomena of the 



