324 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
water on the verandah of the laboratory, but not in getting 
it to continue its apical growth in such circumstances. 
The sensitiveness of the thallus to contact with the 
substratum is very marked, and the thallus follows every 
irregularity of the rock on which it grows. As, on the 
whole, the growing points tend more to grow upwards than to 
grow in other directions, it may be perhaps concluded that 
they have a certain sensitiveness to light or gravity. The 
growing point itself shows a marked dorsi ventral structure, 
but the flower which terminates it does not, though it is 
developr^d in a more or less horizontal position. Corres- 
ponding to its radial symmetry of structure, it no sooner 
escapes from the cupule than it turns upwards, and assumes 
a nearly erect position. 
Though adhering very closely to the rock, the plant does 
not seem dependent on its root-hairs for food, and it is doubt- 
ful if it absorb much from the rock, unless it obtains from 
it the great quantity of silica with which its cells are filled. 
The use of this silica is questionable, but probably it has some 
secondary value in preventing the too rapid drying up of 
the thallus when exposed to the air. Assimilation probably 
goes on in both leaves and thallus, and the relative areas 
would indicate that there is more in the latter. At an early 
period in the life-history, the thallus contains a very large 
quantity of starch, and later in the year, when the flowering 
shoots are developing, the amount stored up at their bases 
and in their tissues is enormous. To this is to be ascribed 
the very great rapidity with which the plant is able to open 
its flowers and to ripen its seeds. 
The deep red colour of the majority of the plants is very 
striking, and when the water is low and the sun shining 
brightly, it is not infrequent to see spots where Weddell’s 
metaphor is not exaggerated, “ le fleuve semble, si l’on me 
pardon l’expression, rouler sur un tapis de roses. ” The 
universality of this colour in the plants of this order, as in 
the deep-growing Florideæ, seems to point to a definite 
physiological meaning of the colour. 
