338 YOUXG SPOROPHYTE OF PIXE 



now apparent why the tube cell extends to the female gamete and 

 also why the archegonial chamber is not developed. The male 

 gametes and the wall cell, now quite unattached, are carried down 

 to the end of the tube (Fig. 254, 10) by the cytoplasmic currents 

 and pass into the cytoplasm of the female gamete through an 

 opening that is formed in the end of the tube. Chemical attrac- 

 tion now draws one of the male gametes to the nucleus of the 

 female and their fusion results in the formation of the gameto- 

 spore. The remaining cells of the male gametophyte are appa- 

 rently disorganized (Fig. 255, A). The functions performed by 

 the tube cell in the Pinales will be a constant characteristic of all 

 the members of the next subdivision or Angiospermae. and it 

 should be noted that this important modification of the male 

 gametophyte probably arose at first as an absorbing organ to 

 supply the antheridial cell with food, as is the case in the cycads. 

 In the more advanced types the archegonia become more effectu- 

 ally enclosed in the sporangial tissues and the tube cell assumed 

 in addition the function of a conduit for the male gametes which 

 lost their motility as a result of this new method of transport to 

 the female gamete. 



(d) Development of the Sporophytc. — The germination of the 

 gametospore of the pine will illustrate the more important fea- 

 tures in the process. The nucleus of the gametospore divides, 

 forming four nuclei (Fig. 255, B), which pass to the upper end 

 of the spore and arrange themselves in a plate. By successive 

 divisions these cells are increased until four plates of four cells 

 each are formed which become surrounded by cell walls save in 

 the case of the lower plate, which is in direct contact with the 

 rich food of the spore (Fig. 255, C-D). This meagre growth 

 corresponds to the pro-embryo of Cycas (Fig. 248, A). In this 

 connection it should be noticed that only a portion of the gameto- 

 spore, as in the cycads, is used in forming the embryo. In all 

 the preceding groups, as the mosses, ferns, etc., the gametospore 

 in germinating behaves as an ordinary cell, dividing into two 

 cells which continue the process. In the Pinales the larger por- 

 tion of the gametospore serves as a storehouse for food, cell for- 

 mation being confined to the upper end of it (Fig. 255, B. ('>. 



