286 Mr. A. l\ W. Thomas. 



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 



