OF CEYLON AND INDIA. 
345 
expected, the floral shoots are simply metamorphosed 
vegetative shoots, and there is no dimorphism in the sense 
ordinarily employed in reference to this plant. Only on 
extremely rare occasions does the whole number of the 
secondary shoots become floriferoiis ; as a rule, about one- 
third or a half of them is the maximum so metamorphosed. 
The non-floriferous shoots are always towards the tip of the 
thallus, the floriferoiis at the lower end, and not infrequently 
some of the creeping thallus becomes floriferous also. 
During this period of submergence the thallus has continued 
to grow in thickness, mainly by meristematic activity in the 
cortex, and it may now be as much as 4 mm. in thickness 
near the base, tapering away to a mere thread towards 
the tip. The leaves of the shoots towards the base usually 
drop as the thallus grows longer, and presently the floral 
shoots are formed at these points. What determines the 
number of shoots that are to become floriferous, unless it be 
the available amount of nutrition, is hard to say ; the boundary 
between the flowering and non-flowering parts of the thallus 
is very sharply marked. In the former the inner collenchy- 
matous cortex becomes very thick-walled, while the thin- 
walled outer cortex shrivels, though it does not fall 
altogether away, as in other Dicræas to be considered 
below. The lower part of the vascular bundle now also 
exhibits the characteristic xylem elements, in the form of 
annular and spiral vessels, while the upper part is phloem 
tissue. Until this time there has of course been no use for 
water-conducting tissue or xylem of any kind, but now the 
floriferous part of the thallus is to come above the water. 
No ligniflcation, however, seems to take place, or at least, the 
walls, though f thickened, give no reaction with aniline 
chloride or phloroglucin. Beyond the floriferous part the 
thickening of the walls of the cortical collenchyma and of 
the vascular strand does not take place. ^ 
The formation of the floral shoots in this genus shows an 
advance on that in the preceding genera with root-thalli, in 
that the whole shoot is floral only, is very short, and bears 
