56 CLASSIFICATION OF PLANTS WITH EEFEEENCE 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 lq 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 in nourishment 

 without the intervention of a cell-membrane). The naked protoplasm in these cases, 

 which include in particular the class of Amcebse, 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 



