400 



GYMNOSPERMS 



There is a single archesporial cell which divides, forming a pri- 

 mary wall cell and megaspore mother-cell. There is no spongy tis- 

 sue, either at this stage or later. The reduction divisions were not 

 observed, but Pearson concluded, from early stages in the female 



gametophy te, that the lower cell 

 of the row develops in the usual 

 way. 



THE MALE GAMETOPHYTE 



The microspore is the first 

 cell of the male gametophyte. 

 Pearson^-'^ followed the devel- 

 opment from the first appear- 

 ance of the archesporial cells up 

 to the shedding of the pollen. 

 The two spore coats, exine and 

 intine, are of about equal thick- 

 ness and usually become more 

 or less split apart (fig. 376). 



The divisions which take place 

 in the microspore before the stage 

 shown in iig. 376, have not been 

 described; but from Pearson's 

 figures and description it is evi- 

 dent that the first division forms 



Fig. 376. — Welwitschia mirabilis: A, 

 three-nucleate stage of pollen grain, X940; 

 B, shedding stage, X600: ex, exine; in, in- 

 tine; «', w^ and n^ interpreted respectively 

 as prothallial, generative, and tube nuclei. 

 — .\ftcr Pkarson.^'' 



a prothaUial nucleus (Pearson says there is certainly no prothalUal 

 cell) and an antheridium initial, and the antheridium initial, at its 

 first division, produces a tube cell and a generative cell. 



In this three-nucleate stage the pollen grain reaches the ovule. 

 The pollination drop, characteristic of gymnosperms, appears at the 

 tip of the projecting micropylar tube, which is the greatly prolonged 

 inner integument. 



Hooker'^' suspected that Welwitschia was insect poUinated; and 

 Baines found a parasitic insect, Odontopus expunctidatus, on plants 

 at Haikamchab. Pearson not only found the insect on both male 

 and female plants, but found pollen on its legs and abdomen. He 

 concluded that it would be impossible for it to crawl over ovulate 



