230 Transactions. 



first by transverse walls arising in the terminal cell (fig. 61), but later it 

 would seem from fig. 49 that oblique walls in the terminal cell cut off seg- 

 ments alternately on one side and on the other, so that the filament becomes 

 two cells in width. All indications that I have noticed point to the fact 

 that this first-formed part of the filament does not subsequently thicken. 

 It is to be found in this thin form still attached at the base of older prothalli 

 (figs. 51, 53, 56), so that it has nothing to do with the " shaft " of the pro- 

 thallus. Presumably the tubercle is formed by the activity of a .single tri- 

 angular apical cell as in Treub's material, though, of course, my material is 

 not young enough to show this. The young prothalli of L. ramulosum given 

 in figs. 63-65 have grown on from the primary tubercle not in a filament 

 one cell wide, but in a shaft of the same width as the tubercle. This shaft 

 becomes progressively wider as it grows on. If a tubercle shows subse- 

 quently a distinct roimded form it must have been due to secondary cell- 

 multiplication, having taken place in it owing to the presence of the fungus. 

 In the prothalli of L. ramulosum shown in figs. 62, 63, and 64 the whole 

 of the small-celled terminal portion is clearly meristematic, so that here, 

 possibly after a period of rest, the whole of the forward end of the tubercle 

 has become meristematic. In the prothallus shown in fig. 61 the original 

 filament gradually thickens as it grows on, apparently by longitudinal 

 walls appearing in the various cells, and the whole of the apex has taken on 

 the function of growth. 



In L. cernuum the apex of the shaft grows on and eventually gives rise 

 to a crown of lobes. Some of Treub's figures seem to indicate that the lobes 

 are initiated when the prothallus is not more than of middle age. The 

 meristematic zone is then largely localized at the base of the lobes, and by 

 its continued activity the lobes are extended in length, and also the length 

 of the shaft is added to. At the same time the shaft can increase slowly 

 in width by the division of its cells in a longitudinal direction. Under 

 certain conditions, perhaps inducing quickness of growth, the shaft becomes 

 somewhat elongated, and the lobes also are well developed (fig. 42). The 

 opposite conditions bring about a thickening of the shaft unaccompanied 

 by any great increase in its length (fig. 44). Thus in the prothallus practi- 

 cally the whole of the main body is more or less meristematic, and the 

 localization of this function at the growing apex is not so definite as 

 in the prothalli of the epiphytic or of the terrestrial subterranean types. 

 Meristematic activity at the base of the lobes becomes most pronoimced 

 when the sexual organs are being formed, so that they are embedded in a 

 small-celled tissue (figs. 40, 45). 



In L. ramulosum the prothallus is capable of much more extensive 

 growth, probably on account of its continued infection by the fungus. At 

 a fairly early stage the apex gives rise to the lobe-like outgrowths, the 

 initiation of which is shown in figs. 63 and 64. In the young prothallus 

 in fig. 65 two outgrowths are well formed and a sexual organ is developing 

 at their base. The lobes are browned and slightly withered, as if they 

 had been touched by a dry spell in the weather. At the same time the 

 shaft of the prothallus is proceeding to extend on past the lobes so that 

 these latter will eventually come to occupy a lateral position. Immediately 

 behind the developing shaft-apex three of the epidermal cells have become 

 infected with the fungus and one epidermal cell has grown out as a 

 rhizoid. It certainly seems as if this fresh infection by the fungus had 

 been the cause of the renewed activity of the prothallus. The study of the 

 prothallus given in fig. 50 suggests that the apical meristem has alternately 



