292 EVANS. 



position a hair has the power of branching at the tip and acting as an 

 organ of attachment. 



The costa shows the same structure as that of M. divaricata, being 

 bounded dorsally by two rows of cortical cells and below by four. The 

 alar cells, according to Stephani, measure 54 X 40 lx, those near the 

 costa being 72 X 40 lx. The writer's measurements give an average of 

 41 X 33 fx and do not indicate that the cells near the costa are appreci- 

 ably longer than the others. The cells throughout have rather firm 

 walls, which often show nodular intermediate thickenings as Stephani 

 notes, but the thickened angles that he likewise emphasizes are diffi- 

 cult to demonstrate. 



No male branches have been seen by the writer and the original 

 description does not mention them. Female branches are often 

 abundantly produced, and it is a noteworthy fact that a female thallus 

 sometimes becomes gemmiparous shortly after it has borne the sexual 

 branches. Some of the latter are small and undeveloped, but most 

 of them are of fair size (Fig. 5, A), measuring perhaps 0.5-0.7 mm. in 

 length and 0.9-1 mm. in width. The outline, which is broadly orbicu- 

 lar with a deep apical indentation, does not show clearly without 

 spreading the branches out flat, owing to their strong concavity. 

 The margin shows crowded hairs borne singly, each representing the 

 outgrowth of a small cell situated between two larger cells, just as in 

 the normal vegetative thalli of most Metzgeriae. On the ventral 

 surface the thickened median portion bears a dense cluster of hairs, 

 and a few other hairs are scattered over the unistratose portion. 



Mention has just been made of gemmiparous plants, although 

 Stephani does not allude to them. As a matter of fact the gemmae of 

 M . patagonica, which are marginal in position, yield some of the most 

 distinctive characters of the species. The gemmiparous branches are 

 at first scarcely modified but rapidly decrease in width after the forma- 

 tion of the gemmae has been initiated. When the wings have been 

 reduced to a width of four or five cells the growth of the branch comes 

 to an end. The reduction in the width of the wings is often accom- 

 panied by a simplification in the structure of the costa, the rows of 

 ventral cortical cells being only two or three. In the formation of 

 the gemmae their mother-cells are derived directly from the marginal 

 cells of the branch, without a preliminary cell-division. The gemmae 

 may be scattered or crowded, a long series of adjoining marginal cells 

 sometimes giving rise to a continuous row of gemmae. The latter 

 tend to appear in acropetal succession and yet show many exceptions 

 to this arrangement. 



