Thomas. — On the Prothallium of Phvlloglossum. 403 



prothallium lives symbiotically. Such a symbiotic fungus 

 has been found in the prothallia of almost all the species of 

 Lycopodium 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 commenced 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 correspond with the oldest prothallium of 

 L. ceriucum obtained in laboratory cultures by Treub. We 

 may take the tuber, which is of constant occurrence, to corre- 

 spond to the primary tubercle seen in the prothallium of 

 L. cemuum. 



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 formation 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-enlarged part of the body of 

 the prothallium. This latter part bears the embryo on one 

 side ; it is evidently formed by the increase of assimilatory 

 tissues for the nutrition of the embryo. Below this swollen 

 part the body contracts again to a cylindrical shaft, which 

 passes downwards, to swell out again and terminate in the 

 primary tubercle. It is this shaft which varies most ; it may 

 be long and narrow, straight or curved, or it may be shorter 

 and stouter, or occasionally, perhaps when the primary 

 tubercle has been formed near the surface of the ground, it 

 may be almost obsolete. The shorter thick-set prothallia 

 may be less than 2 mm. long ; others may range up to thrice 

 this, according to the length of the shaft. Bhizoids are pro- 

 duced in considerable numbers from the lower part of the 

 prothallium, more especially from the tubercle. 



The whole of the upper part of the mature prothallium is 

 green, except the archegonial necks, but the minute chloro- 

 plasts are most abundant in the part below the crown. The 

 cells of the shaft may contain chloroplasts, but the green 

 colour passes away as we follow it downwards into the soil. 



