284 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
water plants live, the Podostemaceæ have constantly and 
rapidly moving water with a rocky bottom, and the water is 
well aerated. The necessity which thus appears to exist for 
many water plants of providing means of supplying oxygen 
for respiration to their more deeply submerged parts is 
absent in the case of the Podostemaceæ. 
Light . — The depth of the water acts in another way upon 
the dis tribution of the plants in the vertical direction 
by regulating their light supply. The vertical range of the 
plants in the Hakinda rapids is small, extending only over 
about three or four feet, or less in many places. The lower 
boundary is mainly determined, it may be well supposed, by 
the amount of light there available to the plants. Possibly 
the actual pressure due to the water may join in producing 
the result, but most probably the light is the larger factor, and 
both depend directly on the depth of the water. During 
periods of high water the plants seem to tend on the whole 
to grow upwards rather than downwards. 
The depth varies considerably at different seasons, so that 
the light supply also varies, and the effect is intensified by the 
fact that the muddiness is usually greater in the deeper water. 
The earliest stages of the life-history must go on in 
comparative darkness (especially as there is less sunshine in 
the wet weather), while in August, September, and early 
October, the period of greatest vegetative activity, the plants 
are exposed to fairly bright light, being at that period close 
to the water surface. With the reserve materials stored up 
during this period the development of the floral shoots 
is ultimately carried out towards the end of the high water- 
level and comparative darkness of the north-east monsoon, 
and reserves are also provided for the ripening of the seeds. 
In the more northern stations of the Western Ghats this 
statement must be modified, for there is there no drier 
period between two wet ones. The life-history in this 
district has yet to be studied, but probably during the time 
of greatest depth of water the plants are vegetative only, 
while they store their reserves and develop their flowers 
