358 Transactions. 



dorsal surface of the " rhizome," and a stem apex is differentiated. In the 

 case of the young plant sho\Yn at Plate XXXI, fig. 8, where the rhizome had 

 branched a stem had developed on each branch. 



Whether (with Bower) we regard the protocorm as being a late specialisa- 

 tion — a " gouty interlude " in the development of the sporophyte from its 

 strobiloid ancestor — or whether, on the other hand, we regard it as being the 

 representative of a possible thalloid ancestor, the case of L. laterale is inter- 

 esting as indicating the important degree of development of which this 

 protocormous organ is capable. In the young plant of L. cernuum the pro- 

 tocorm is seen to function only as a very temporary organ, important only 

 during the short period which intervenes between the stage at which the 

 young sporophyte has begun to grow beyond the supply of food contained 

 in its prothallus and the point of development of its own regular assimilatory 

 system of stem leaves and root. In this species, although the stem axis 

 and first root are not differentiated at a very early stage, they, at any rate, 

 make their appearance very much earlier than in the case of L. laterale. In 

 this latter species the protocormous " rhizome " attains a considerable size, 

 and constitutes the plant body for a long period, and by no means bears 

 the charactei: of a mere temporary organ. The stem and first root are here 

 very late in development. It might seem at first sight that the case of 

 L. laterale affords an argument against the theory of the late phylogenetic 

 development of the " protocorm," favouring rather that other theory 

 which would regard it as the very much reduced representative of a 

 thalloid body, the homologue of the " prothallus." It must, however, be 

 borne in mind that with considerable reason it may be urged that the 

 protocormous rhizome of L. laterale is merely a physiological specialisation 

 peculiar to this species, designed to carry the young plant over a season 

 unsuitable for its further development. In this connection we observe 

 that the usual habitat of L. laterale is a marshy one, and that the rhizome 

 is stored with starch and water. 



In the examination of the young plants of the remaining four species, no 

 structure comparable to the protocorm was found. 



Foot. 



A series of longitudinal microtome sections of the young plant of 

 L. cernuum, shown at Plate XXXI, fig. 1, was taken, and this revealed the 

 fact that the " foot " was exceedingly small. 



In the case of L. volubile, L. scariosuin, L. densum, and L. Billardieri 

 the foot was always large and very persistent, being recognisable on the 

 young plant long after the prothallus had decayed away. In L. cernuum 

 it consisted of a very few similar parenchymatous cells ; but in the latter 

 four species the layer of cells of the foot adjacent to the prothallial tissues 

 was developed as a distinct epithelial layer (see Plate XXXI, fig. 17), whilst 

 the more centrally placed cells were elongated slightly in the direction in 

 which food substances would pass from the prothallus into the vascular 

 strand of the young plant. The vascular tissues of the stem in these species 

 did not enter the foot, but passed directly into the root. 



First Root. 



In the young plants of L. cernumn, about the same time as the stem apex 

 is differentiated, the first root makes its appearance as a superficial out- 

 growth of the upper region of the protocorm (Plate XXXI, fig. 2), into which 



