Page:Proceedings of the Royal Society of London Vol 69.djvu/302

286 ordinary leaves of Lycopods. Further, the embryo of L. cernuum forms at a very early stage a tuber (protocorm), above which its proto- phylls rise. To this protocorm the tuber of Phylloglossum is appa- rently comparable, but in Phylloglossum it is not a passing embryonic structure, but is repeated annually on the formation of a new proto- corm. Treub regarded the protocorm as the representative of a primitive structure originally possessed by the Pteridophytes, a struc- ture which may have served an important part in the phylogeny of the higher plants, in enabling the sporophyte to attain an existence inde- pendent of the gametophyte.

The prothallia of Phylloglossum have been obtained growing naturally amongst the parent plants, but it is a significant fact that in most places, even where. older plants are abundant enough, no prothallia could be discovered, though many whole days were spent in fruitless search. In three localities only were prothalli discoverable. It appears clear that very special conditions are necessary for the germination of the spores, conditions which are not of regular annual recurrence wherever Phylloglossum grows. Perhaps the most im- portant of these conditions is the presence of a fungus with which the prothallium lives symbiotically. Such a symbiotic fungus has been found in the prothallia of all the species of Lycopodiiim in which the development is known.

Before describing the prothallia it will be well to state that they vary remarkably in external form. Such variations as depend upon the stage of development present no difficulty, but there are other differences which are probably accidental, being due to obstacles in the soil or to the depth beneath the surface at which the prothallium com- menced its development.

One of the simplest and perhaps the youngest observed, consisted of an oval tuber below, from which rose a simple cylindrical shaft with rounded apex. Such a prothallium appears to closely corre- spond with the oldest prothallium of L. cernuum obtained in labora- tory cultures by Treub. We may take the tuber, which is of constant occurrence, to correspond to the primary tubercle seen in the prothal- lium of L. cernuum.

A. more advanced prothallium shows the cylindrical part of greater length and thickness, and its end slightly expanded into a crown, on which the first sexual organs appear. A little below the crown the tissues of the cylindrical body are conspicuously meristematic, especially on one side. This meristem lies below the archegonia, and its forma- tion appears to be anticipatory of the descent of the embryo.

Older prothallia in which an embryo is already present are much more irregular in form. The crown, which may be conical, rounded, or projecting to one side, and then often shaped like the head of a horse, is commonly separated by a slight constriction from the much