134 A. H. Graves, 



on this condition in Lemna, say, "To divide a large sporogenous 

 mass by sterile plates for better nutrition is too common to call 

 for special remark." As mentioned by Caldwell and Goebel 

 (1898, p. 770), Isoetes presents a similar condition of formation of 

 sterile plates of tissue from a fairly large archesporial mass.^ 



The archesporial cells, therefore, now appear as two definite, 

 rounded, densely staining masses, composed of sporogenous cells, 

 surrounded by a primary parietal layer, which has undergone a 

 periclinal division in two or three places. 



The first periclinal divisions in the primar}- parietal layer have 

 become more general in the next stage, PL XI, fig. 73, which is from 

 a flower about 0.33 mm. in length. The septum between the two 

 sacs is also more conspicuous, and divisions continue in the sporo- 

 genous tissue. 



At a considerably later period, with the length of the flower 

 about 0.5 mm. (PI. XI, fig. 74), the parietal layers are still two or 

 occasionally three in number. Indications appear here that the ta- 

 petum is forming from the marginal sporogenous tissue. Nuclear 

 divisions continue among the sporogenous cells. 



Soon after this stage, however, the sporogenous cells attain their 

 final number, and all division ceases, followed by an enlargement 

 to the mature pollen mother-cells, just as Murbeck (1902) has re- 

 corded for Ruppia rostellata. 



PL XI, fig. 75 shows how the tapetal cells, now unmistakable in 

 form and structure, bound the sporogenous cells — which may now 

 be termed the pollen mother-cells — and are undoubtedly derived 

 from them. According to Rosenberg (1901, 11), Zostera also forms 

 tapetum from the ends of its long sporogenous cells, and Coulter 

 and Chamberlain (1903. p. 37) have shown that this is not unusual 

 nor unnatural. In this respect, together with the number of chro- 

 mosomes in the dividing sporogenous cells, which I have found to 

 be 16, and also the three or four parietal layers between the epi- 

 dermis and tapetum, R. ntaritima corresponds exactly with R. ro- 

 stellata, as described b}' Murbeck (1902, p. 5). It is interesting to 

 note here that Wiegand (1899, pp. 344 and 345), finds in Potamogeton 

 foliosus that the tapetum is " diff"erentiated from the wall rather 

 than from the archesporium." 



There remain to be mentioned the dissolution of the tapetal cells 

 (PL XI, fig. 76), the development of thickenings in the subepidermal 

 layer, and the final dehiscing of the anther by a longitudinal split. 



1 A like situation has been carefulty described by Bower (1897, pp. 41 ff .) 

 for Danaea and other Marattiaceae. 



I 



