294 Evans .— Vegetative Reproduction in Metzgeria. 
the substratum, and may in time assume a position at right angles to it. 
The hairs become fewer and fewer, and before long cease to be developed at 
all, and the slime papillae also fail to appear. The wings become strongly 
convex, and at the same time grow more and more narrow until in some 
cases the thallus becomes reduced to the costa. The first change to be 
noted in the costa is a reduction in the number of internal cells (Fig. 16, b). 
As growth continues the number of these cells again becomes larger 
(Fig. 16, c). With their increase in number they also increase in size, thus 
approaching in appearance the cortical cells, but they can always be 
Fig. 16. M. fruticulosa. A. Cross section of a normal thallus. x8o. b. Section through 
the lower portion of a gemmiparous branch, x 300. C. Section through the upper portion of 
a gemmiparous branch, x 300. d. Young gemmae, x 300. e, f. Gemmae about ready to 
separate, x 80. 
distinguished by their somewhat greater length. The cortical cells first 
remain in four rows, but are more bulging than in a normal costa. 
Later on they become more numerous, forming perhaps four rows both 
antically and postically, but as they increase in number their arrangement 
becomes irregular so that the rows are no longer distinct. If the wings 
become reduced to the cells which bound the internal tissues of the costa 
laterally, then the gemmiparous axis exhibits a condition of radial sym¬ 
metry, as Goebel (’98 a , p. 275) has described for M. conjugata. The radial 
portion of the shoot is always very short because its growth comes 
to an end soon after extreme specialization has been reached. Very 
often, in fact, the growth stops before the radial condition has been 
attained. 
