202 Transactions. 



which are not median the palisade tissue presents this appearance, in 

 sections where it is cut exactly longitudinally it is found to be only one 

 cell thick. These cells are exceedingly long and narrow, and occasionally 

 divide transversely. Again, Miss Edgerlev describes the cell-layer of the 

 central core which abuts on to the palisade tissue as containing abundant 

 starch and acting as a storage tissue. I was not able to find this in any 

 of ray sections, but, as in the case of the prothallus of L. volubile, this may 

 be due to the fact that our specimens were collected at different seasons. 

 On the whole, the main feature in which the prothallus of L. scariosum 

 differs from that of L. coniplanatum is in the large, irregular size attained 

 by the mature individual. Young or even half-grown prothalli sometimes 

 show the tapering carrot form, but sooner or later this becomes modified 

 owing to the enormous development of the centrally placed core-cells, which 

 evidently must function as a store tissue. However, even very young 

 prothalli sometimes show a rounded lower vegetative region instead of 

 the tapering one, the first-formed conical region being in these cases quite 

 blunt. This difference, then, is to be referred to the manner of development 

 of the prothallus from the beginning. Whereas in L. coniplanatum the 

 increase in girth is very gradual, in L. scariosum it is generally rapid from 

 the actual point upwards. This may be occasioned possibly by the early 

 development of the storage function of the central core of cells, or it 

 may be due to a deeper constitutional difference between the two types. 

 In this connection, however, it is significant to note that the type 

 of heterophylly in both species is alike. In Part I of these Studies 

 (16, figs. 49-52) I have figured several very irregularly grown large pro- 

 thalli. As Miss Edgerley notes, the most striking feature in the large 

 prothallus of this species is the relatively small proportion of the bulk of 

 the vegetative region which is occupied by the fungal tissues, a feature 

 in which it is strikingly different from that of L. volubile and L. fastigiatum. 



It will thus be seen from a comparison of the prothalli of the species 

 mentioned that as regards their fungal zones the two types are not very 

 dissimilar. There are two main points of difference to be noticed : first, 

 the increase of girth of the clavatum type from the original point upwards 

 is more rapid, and hence the height of the prothallus is less than in the case 

 of the complanaimn type ; and, secondly, the extent of the upper generative 

 tissue in L. clavatum is greater than in the latter species, these two features 

 being closely interdependent. The fact that in L. clavatum the upper 

 generative region is more or less saucer-shaped at its surface, with a dis- 

 tinct rim, and is often accompanied in large specimens by a development 

 of warty protuberances, whereas in L. complanatum it takes the form of 

 a compact semicircular crown, would seem to be simply the physical result 

 of the two types of growth. Lang suggests that " the flattened and still 

 more the trough-like form which these older prothalli present may be an 

 adaptation to facilitate fertilization " (24, p. 296). There is no doubt that 

 this particular shape does serve this purpose, but I would be inclined to 

 explain the difference in form between the two types in rather a different 

 way. I have found that the prothalli of L. scariosmn always occur at a 

 greater depth than those of L. voluhile and L. fastigiatum. In localities 

 where the prothallus of L. scariosum and L. fastigiatum were growing 

 together I always unearthed the latter from the layer of humus which 

 immediately underlay the carpet of moss, &c., whereas the former had to be 

 dug out of the deeper-lying clay. Not infrequently, too, I have found 

 the prothalli both of L. volubile and of L. fastigiatum amongst the thick 



