304 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
the shoots, instead of being, as sometimes is the case with 
such adventitious development of lateral roots, intended 
mainly for anchorage or absorption. There is no great 
assimilatory function performed in the thallus in this 
genus, bu,t we get a distinct stage on the way to such cases as 
Dicræa or Hydrobryum. Anatomically, almost more than 
morphologically, the root shows great dorsiventrality. 
In the haptera, we meet with organs for which there 
appears to be no precedent in other water plants. Their 
function is evidently to act as holdfasts, and they may 
therefore be regarded as adaptations to the peculiar mode of 
life, except that, so far as we know, they are not hereditary 
in the strict sense, but are developed apparently in response 
to a direct stimulus, at any point where they are required. 
Their size and form also seem to be directly dependent on 
the mechanical conditions of life. 
The secondary shoots are endogenously developed on the 
root-thalli, and here we meet with one of the most marked 
general features of the whole order. Such formation of 
secondary shoots upon roots is not entirely unknown else- 
where, but it is not carried to the extreme pitch of frequency, 
regularity, and complexity that is found in this family. 
In the absence of other families of plants living under 
similar conditions of life, or showing this peculiar feature 
under different conditions, it is difficult to decide whether 
this development of numerous secondary shoots is really an 
adaptation to the conditions of life or not, but I am inclined 
to think that it is. In this place, however, it is sufficient to 
point out its advantages, in that it reduces the risk of ex- 
termination when a temporary fall of water occurs other 
than at the regular flowering season, which, if there were 
only one axis, would be very great. 
The secondary shoots are large and complex, but have an 
evident aquatic habit and structure ; the vascular tissue is 
reduced and central, and there is great development of 
green assimilating tissue by means of the formation of the 
ramuli. 
