1905. ] On the Cytology of Apogamy and Apospory. 465 
growth caused by the division both of the marginal cells of the leaf and of 
those cells lying immediately within the margin. As the growth proceeds, it 
is clearly distinguished as a more or less continuous sheet of delicate tissue 
formed of somewhat rectangular cells. In due course, owing to rapid growth 
at certain points, the typical prothalloid shape is assumed. The study of 
many pinnules shows the apex either produced into a single prothallus (fig. 2) 

v.b. 
Fic. 2.—Prothallus grown from tip of pinnule. p., prothallus, pin., pinnule, 
v.b., vascular bundle. 
or crowned by a cluster of prothalli, or else the edge is beset with isolated 
groups. The prothalli on the surface of the leaf may be closely approximated 
to, or independent of, the vascular tissue. The majority of the prothalli are 
typically regular in shape, but irregular ones are by no means uncommon. 
The regular prothalli are normal in appearance, except that they have no 
well-developed cushion. Antheridia are frequently found, even when the 
prothalli are comparatively young, but archegonia have never been seen. The 
prothalli generally show the presence of an embryo in various stages of 
erowth. It is situated in the position normally occupied by the cushion. The 
irregular prothalli already alluded to vary greatly in shape, and apparently 
seldom bear antheridia, and never give rise to an embryo. 
The principal cytological interest in the prothalli centres in the number 
of the chromosomes, and in a comparison between the prothallus and the 
212 
