AQUATIC PLANTS. 
69 
oxygen is now absorbed by the fluids of the tissue and gradually transformed into 
carbon-dioxide, which can again diffuse back into the cavities, but partially also 
through the layers of tissue into the surrounding water. This, as well as the different 
coetHcients of diffusion of the gases, causes the air contained in the cavities to have 
an altogether diiferent composition from that in solution in the surrounding water, 
and this composition to be subject to continual change, as also will be the pressure. 
The carbon-dioxide taken in by aquatic plants is partly dissolved in the 
water as such, and partly as calcium-bicarbonate. The plant taking in 
part of the carbon-dioxide in this bicarbonate converts it into the insoluble 
monocarbonate, which is thereupon thrown down as an incrustation on the 
surface of the plant. This is the origin of the calcareous covering alike of 
Nullipores in salt water and of the Characete and others in fresh water. 
The much-branched character of the stems, and still more of the 
leaves, of water-plants serves to expose a maximum of surface to the 
surrounding nutrient medium, though deficiency of light and mechanical 
considerations are also important contributing causes for the dissected 
submerged leaves of the numerous heterophyllous aquatics, such as 
■Salvinia, Trapa, Cabomba, and the Batrachian Banunculi. This 
■dissection of the leaf is commonly associated with the absence or reduc- 
tion of the roots. Thus the rootless Salvinia, one of the Hydropterideae, 
and the widely separated Water-chestnut, Trapa, allied to the Onagraccce, 
agree in having hair-like divisions of submerged leaves which resemble 
roots and serve undoubtedly to absorb the liquid food, while the floating 
leaves assimilate. Biccia, Wolffia, Ceratojjhyllum, Hydrocharis, and 
Aldrovanda, wide apart systematically, are all rootless, and Utricularia is 
so even in the embryo. Though possessing roots, Butomus, Caltha, 
Hippuris, Myriopliylliim, Menyanthes, Castalia, and Leinna are all 
destitute of root-hairs. As, however, one of the main functions of roots 
is to take in the water required for transpiration, this absence of root is 
connected as much with the absence of transpiration in submerged 
structures as it is with the cognate feature of absorption of dissolved 
gaseous and saline food through the whole surface. The numerous 
water-plants that are not free-swimming and rootless, but are anchored 
to rocks or mud, such as Fucus and many FloridccB, Isoetes, Pilularia, 
Fotamogeton, Zostem, Elodea, Vallisneria, and many others, depend 
upon their roots for little beyond the mere attachment,! absorbing saline 
food in large proportions by their whole surface, which is often specially 
adapted for this function. Thus fresh water when *' soft," i.e. free from 
lime and iron salts, is poor in vegetable life : marine Algae are well known 
to take in large proportions of soda and iodine, substances that are not 
physiologically essential to them, their ash, in consequence, exceeding 
that of land plants ; and no plant ash yields on analysis such high per- 
centages of iron as those of Trapa and Lcmna.^' 
Transpiration, or the passing-off of water vapour from the plant into 
the air, though a function which is retained by floating leaves is absent 
from submerged parts, and its absence entails most important histo- 
logical changes. Thus in floating leaves, such as those of NyiupJiwa,"^ 
* 7-4 per cent, in the ash of Lemna, 29-6 in that of the leaves, and OS'G per cent, 
in that of the pericarp of Trapa natans. 
f I employ Linna,us' generic name for the Yellow Water-lilios, often named Xuphar, 
using, according to the strict law of priority, Salisbury's generic name. Castalia. for 
the White Water-lily and its allies. 
