I04 Alexander W. Evans, 



tissue in the form of a hollow cylinder and composed of long cells, 

 usually pointed at the ends, with thick and often pigmented walls. 

 A cylinder of this type was first descrilsed by Leitgeb in the case 

 of R. eriocaula (15, p. 49). It is never superficial in position but 

 is bounded on the outside by one or two layers of short cells with 

 thinner walls, the diameter of which often equals or exceeds that 

 of the sclerotic cells. On the inside the sclerotic cylinder passes 

 by a gradual or abrupt transition into a central core composed of 

 paler and often colorless cells with thinner walls. In the pho- 

 tosynthetic branches the sclerotic tissue persists in a reduced form, 

 the cells being fewer and having more delicate walls ; in the ulti- 

 mate branches these cells usually occupy the entire interior of the 

 thickened median portion, the central core of the axis having 

 tapered to an end in the larger branches. The cells which form 

 the unistratose wings of typical photosynthetic branches and also 

 the surface-cells of the median portion are usually broader than 

 the interior cells and often show distinct trigones. In a general 

 way the stolons resemble the main axis but the sclerotic tissue is 

 only vaguely indicated. In accordance with their function the 

 stolons usually produce rhizoids in abundance, and the remaining 

 parts of the thallus may be entirely free from them. 



Another type of cell-differentiation is shown by species which 

 produce special appendicular outgrowths or organs, in addition to 

 the rhizoids and slime papillae. The simplest of these, too simple 

 perhaps to be called outgrowths at all, are in the form of local wall- 

 thickenings of the surface-cells. They may be exceedingly irregu- 

 lar or may give rise to low anastomosing ridges, having the effect 

 of teeth when seen in profile. Such thickenings are found in the 

 Javan R. scahra Schiffn. (20, p. 41) and in Stephani's Aneura 

 stolonifcra of Australia and New Zealand (see 25, p. 265, pi. 

 26, f. 3). Among the Chilean species J^. gramilafa shows them in 

 an unusually pronounced form (Fig. 13. F-H). 



Somewhat more definite than the wall-thickenings are the low 

 outgrowths formed by the projections of individual cells. On the 

 margin these cells may give the effect of crenulations or denticula- 

 tions, as in R. mulfifida and R. TJiaxtcri (see Fig. 2, H) ; on the 

 surface, as in R. Jiynnenophylloidcs, they produce a slightly 

 roughened appearance and doubtless play a part, as suggested by 

 Goebel, in the retention of water. Outgrowths of this type are 



