56 CLASSIFICATION OF PLANTS WITH REFERENCE TO NUTRITION. 
line of demarcation exists between these groups; all are connected by numerous 
intermediate links, and there are forms which belong to one group at one stage of 
development and to another at another stage. 
The distinctive property of aquatic plants is that they derive their nourishment 
either entirely or principally from the surrounding water. Some preserve their 
freedom, floating or swimming about in the liquid medium; but the majority are 
fixed somewhere under the water by special organs of attachment. Many plants 
that are rooted in the mud at the bottom of pools are able to derive their food from 
the water when it is high, and when it is low, from the atmosphere as well: such 
amphibious organisms form a transitional group between water-plants and land- 
plants. The number of lithophytes is comparatively very small. They include 
those lichens and mosses which cling in immediate contact to the surface of 
stones and derive their food in a fluid state direct from the atmosphere. All 
lithophytes are so constituted that they can, without injury, dry up and suspend 
their vitality for a time when there is a failure of atmospheric precipitation lasting 
over a long period or when the air itself is very dry. But not every plant which 
grows upon rocks is to be regarded as a lithophyte in the narrower acceptation of 
the term. Those that are rooted in earth in the cracks and crevices of the rock 
must be classed amongst land-plants. To this class indeed more than half the 
plants now in existence belong. Though surrounded by air as regards a part of 
their structure they have another part sunk in the soil, and from the soil they take 
up water and inorganic compounds in aqueous solution. Plants which grow attached 
to other plants or to animals are called epiphytes. 
The majority of plants are during the period of food-absorption connected with 
the foster-earth and are not capable of locomotion. The plant being fixed to one 
spot must therefore sooner or later exhaust the ground in its neighbourhood, and 
must require a further supply of nutritive substances. The parts specially devoted 
to food-absorption often lengthen out in these circumstances beyond the im- 
poverished region, and thus endeavour to bring areas more and more distant within 
the range of absorption. Many plants possess the faculty, to which reference has 
already been made, of alluring animals and of killing and sucking their juices. Not 
only amongst saprophytes and parasites, but also amongst aquatic plants, instances 
occur in which certain movements are performed involving the whole body of the 
organism, with a view to promoting the absorption of nutriment. Particularly striking 
in this respect are many plasmoid fungi (which we may well refer to here, not on 
this account alone, but also for the additional reason that they take im nourishment 
without the intervention of a cell-membrane). The naked protoplasm in these cases, 
which include in particular the class of Amcebze, crawls in its search for food over 
the nourishing substratum, and derives from it immediately the materials needful for 
growth. Loose bodies are liable to be seized by the radiating processes of the proto- 
plasm, which then closes round them and drains them completely of their juices (see 
fig. 9, the last figure to the right). These bodies encompassed by the protoplasm, if 
small, are drawn inwards from the periphery and are regularly digested in the 
