1920 Pease; on Desmarestia 343 



jDarts of the thallus, aside from increase in size of the original grouiw* 

 tissue eells^ is brought about, as stated before, by secondary growth, that 

 is, by continued development of conducting hyphae. 



A longitudinal section through the upper part of the stipe of a mature 

 plant is shown in fig. 4. Contrast the length of the portion of an axial 

 cell shown with the length of the axial cells in fig. 5, a longitudinal section 

 through the base of the stipe and upper part of the holdfast in a j^oung 

 plant. Note also in fig. 5 the mass of conducting hyphae surrounding the 

 central axis. In this young plant an inner assimilation tissue had not yet 

 developed, though sections of the same region in mature plants show an 

 abundant development of assimilating cells. 



In discussing the structure of his D. tahacoides, Okamura (1908) 

 says, "The axis is not composed of a single longitudinal row of cylindrical 

 cells, but of many short filamentous cells, densely packed and firmly 

 coalesced in a very irregular manner." This would correspond to the 

 "tertiary assimilation tissue" which Jonnson (1901) describes in D. aciil- 

 eata, formed by outgrowths from tlie secondary assimilation tissue, or 

 "inner assimilation" of "Soderstrom" (1889), which pierce the walls of 

 the axial cells and form dense masses in the cell cavities. The writer has 

 found nothing resembling a development of this sort in any of the ligulate 

 species examined. 



g. Conclusions 



In the four ligulate species of the genus Desmarestia under discussion, 

 the difference in width and thickness, in venation, and in texture of the 

 thallus is due, not to fundamental differences in structure, but to differ- 

 ences in the relative amounts of primary and secondary tissues, and to 

 differences in the size of the cells of corresponding tissues and the thick- 

 ness of their walls. A comparison of the figures in plates 59-61 will make 

 this very evident. 



In D. ligulata there is comparatively little lateral extension of the 

 lamina and an abundant development of conducting hyphae, making a 

 thick and narrow, but very tough and pliable tliallus. In D. herhacca 

 there is greater lateral extension but less development of hyphal tissue, 

 and hence a broader but much more delicate lamina. In 1). latissima the 

 lamina is enormously extended, there is comparatively little hy])hal tissue, 

 except in the basal portions, but the individual cells are extremely large 

 and thick-walled. There is also an extreme develoiinunt of secondary 

 branches of the inner assimilation tissue. In D. foliacca the extreme thin 

 ness of the lamina is due to the fact that it is the miniinuin iiuinbcr of cells 

 in thickness, and the cells are very thin walled, especially in younger 

 plants, so that when dry there is ver}^ little substance. 



