1896-97-] THE GAMETOPHYTE OF BOTRYCHIUM VIRGINIANL'M. 285 



leafstalk is forced continually outwards and eventually decays, leaving 

 no trace of its existence. This is the reason that, in transverse sections 

 of older stems, the foliar bundles of fallen leaves apparently disappear 

 before reaching the external cortex. The periderm formation of B. 

 virginianuui is thus connected with the occlusion of the leafstalks, and 

 is probably to be explained as an adaptation for protecting the subter- 

 ranean stem from infection by the fungi of the soil. 



In a transverse section through the older region of the stem, the 

 periderm is never found to form a continuous investiture as in the higher 

 plants, but is strictly localized in areas representing the points of origin 

 of former leaves. The writer has not yet had an opportunity of inves- 

 tigating whether the mode of cork formation obtaining in B.virginianum 

 is common to the whole group, but it seems probable that this may 

 prove to be the case. Periderm is also often formed both in the sporo- 

 phyte and in the gametophyte where surface injuries have occurred : a 

 striking case of correspondence between the two generations. 



The cotyledonary trace originates from the central cylinder as a single 

 strand, figure 6l, cot. ; but separates shortly after reaching the petiole into 

 two approximately collateral bundles. These pass upwards through the 

 long leafstalk into the lateral lobes of the lamina, one of them giving off" 

 a bundle for the median lobe, exactly as in the postcotyledonary leaves 

 of many Filicinece. The endodermis is never quite continuous on the 

 inner side of the cotyledonary trace, and in subsequent leaves becomes 

 less and less marked, till at the stage in which there are four petiolar 

 bundles, it is entirely absent. Figure 6"] represents the laminar portion 

 of the ninth leaf of a sporophyte which was still attached to its prothal- 

 lium. The fertile segment,/! s., of the lamina is already present. This 

 plant was at the same time the oldest sporophyte still in connection with 

 the gametophyte, and the youngest already producing spores, which has 

 come under my notice during the present investigation. 



In figure 68 is a still attached young sporophyte. Its prothallium is 

 infected with the already defunct symbiont, a. The spore-plant still 

 bears its cotyledon /;• and two younger leaves,/^ and l^ are in the process 

 of formation. In the primitive root, r, can be seen at ;t'and j, certain 

 dark spots which are cells occupied by the sporophytic endophyte. 

 There is no resemblance between the latter and that of the gametophyte 

 as its mycelial filaments are much larger, being generally about eight 

 micra in diameter. There are no vesicles nor conidia present, and in fact 

 the sterile uiyceliuni is uniformly filamentous in character. These features 

 are reproduced in figure 69. The occurrence of a symbiont in the roots 



