1920 Pease; on Desmarestia 311 



e. Desmarestia latissima Setchell and Gardner in Litt. n. sp. 



(Plates 55, 56, 57; Plate 58, Figs. 1-4; Plate 60, Figs. 5-9; 

 Plate 62, Figs. 8-10.) 



Several very young plants from one to five centimeters long were 

 collected, one of which still bore a few assimilating hairs. Not enough 

 hairs were observed to make general statements possible. Those studied 

 were small and sparsely branched ; their main axis spindle-shaped and 

 less than 20 cells long; their branches either single or in pairs, tapering 

 at the ends, and composed of from 5 tO' 8 cells. Several pinnules termin- 

 ated in unbranched hairs. 



As in D. ligulata every cell of the permanent central axis, laid down 

 below the intercalary growing point, bears a pair of lateral branches. By 

 subsequent repeated divisions of the original axial cells, these paired 

 branches become separated by a series of up to 8 cells, whereas, in D. ligulata 

 not more than 4 axial cells were observed between successive pairs of 

 branches. The cells of the axial row could be counted very easily by focus- 

 ing down through the superficial cells of entire j'oung plants which had 

 been lightly stained in Bismarck brown. 



The young tips of the branches were in the same stages of develop- 

 ment as the tips of the pinnules in D. ligulata. Cortication proceeds in the 

 same way. At the tips of those branches whose axial cells did not pro- 

 duce hair branches, a pair of lateral cells was cut off, as in the beginning 

 of branch hairs, but these single cells cut off small cells below, then at the 

 side, and cortication proceeded in the usual way, b}^ repeated division of 

 these cells. 



In these same plants the beginnings of the inner assimilation tissues 

 were visible as slender filaments made up of cylindrical cells, arising in 

 the cells of the ground tissues and closely applied to the cells of the central 

 axis. Secondary growth from the lateral branches of the central axis was 

 also well under way, the secondary branches themselves branching and 

 ramifying among the cells of the ground tissue. 



All these cells in these young plants were extremely small, and lateral 

 branches were very close together. Young plants up to 50 cm. in length 

 bore no greater number of branches, hence a tremendous elongation of 

 the cells of the thallus between the branches must take place. 



The structure of the mature thallus does not differ from that of 

 D. herbacea except in degree. The lamina consists of the same five tissue 

 regions, developed to a much greater extent, as the enormous widtli of 



