CLVIIL— THE WATER STARWORT. 
Callitriche palustris Linne. 
I F the affinities of apetalous Families of plants are difficult to trace, those of many 
groups of apetalous aquatic plants are even more so. The chief interest, in fact, 
which aquatic plants have for the physiologist is their illustration of the production 
of resemblances in general habit, or in the form and structure of the vegetative organs, 
of plants of the most varied ancestry, by the action of closely similar conditions of 
life. Though belonging to many Orders widely divergent in their affinities, as 
indicated more or less by their reproductive organs, aquatic plants thus form what 
is termed a biological group. 
Its high specific and latent heats render water so much slower than air to change 
its temperature that aquatic plants are but little liable to have their growth or their 
diffusion checked by such changes. They are thus mostly perennials and of very 
wide distribution, at least within the same zone of temperature. Their perennial 
habit makes those of Temperate latitudes where frost occurs require some method of 
hibernation ; and the Water Starworts ( Callitriche ) are among the few which remain 
unaltered and do not sink to the bottom before the winter. Water, again, offers 
considerable obstruction to the transmission of light ; and more or less submerged 
plants have accordingly the long internodes and the thin leaves with green 
colouring-matter in their surface-cells which are characteristic of shade plants. It is 
probably mainly the demand for light that brings about the ribbon-like elongation 
of some submerged leaves ; and the presence of a dense rosette of floating leaves 
will obviously intensify the shade effect. It is interesting to note in Callilriche that 
the submerged leaves are longer and narrower the deeper they are below the 
surface. 
Aquatic Flowering Plants, however, show themselves to be derived from land 
forms by the fact that, except in a very few cases, they do not flower under water ; 
nor is their pollen often specially adapted for conveyance by water. When we 
find a whole genus or Family possessing the aquatic habit we conclude that it is of 
more ancient origin than where — as in Ranunculus — only some species have it ; and 
it is especially in these cases that the affinities of the groups are so difficult to trace. 
The Family Callitrichacece contains only the one genus Callitriche , so named by 
Pliny (from the Greek jcaXos, halos , beautiful, and Opii;, thrix , hair) with reference to 
its slender hair-like stems, or, perhaps, to the silvery, colourless, and transparent roots 
which descend from their nodes. Some twenty-five species have been described ; 
but it is difficult to avoid the conclusion that the plants vary much under the 
influence of slightly differing conditions, so that the real number of species is much 
smaller. Thus eight British forms have been named, including those inhabiting 
mud, deep lakes, and shallow ponds ; and one of the most marked distinctions 
between them is the presence of a rosette of floating leaves, or its absence. 
