ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 91 



ever-moist hollows and depressions in proximity to springs (e.g. Saxifraga aizoides 

 and many others). But wherever the substances to be absorbed are not so easily 

 obtained, the surfaces of the absorptive cells are increased by means of a protrusion 

 of the outer cell-wall, the whole cell being converted into a tube. These tubular 

 absorptive cells are most elongated in mossy forests, where rather large gaps occur 

 not infrequently in the soil. When a root in the course of growth reaches one of 

 these lacuna?, filled with moist air, its root-hairs often lengthen out to an extraordi- 

 nary extent, and sometimes attain to twice the length of those which are in compact 

 soil. The absorptive cells on the roots of the Water-hemlock (Cicuta virosa) and 

 the Sweet Flag (Acorus Calamus) do not project at all if the earth in which they 

 grow is muddy; whilst, if the earth is only slightly damp, and an increase of surface 

 is therefore advantageous, the absorptive cells become tubular. Plants which grow 

 in ground liable to periodic drought, and which at these times must secure all the 

 moisture retained by the earth to save their aerial portions from death by desiccation, 

 endeavour to obtain as great an area of absorption as possible by the development 

 of long tubular cells. 



The fact must not be overlooked, however, that the form and development of 

 absorptive cells depend partly on the quantity of water that is given off from the 

 aerial parts of the plant, that is to say, by the transpiration of the foliage-leaves. 

 Plants which lose a great deal of water in this way must provide for abundant resti- 

 tution. They must absorb from as large an area as possible, and enlarge their absorp- 

 tive surfaces adequately by pushing out the cells into long tubes. For this reason 

 all plants with very thin, delicate, expanded foliage-leaves, which transpire readily 

 and abundantly, have numerous long tubular root-hairs. Examples are afforded by 

 Viola bijiora and the various species of Impatiens. On the other hand, plants with 

 stiff, leathery leaves, being protected by a thick epidermis from excessive transpira- 

 tion, as, for instance, the Date-palm, exhibit flat, non-protuberant absorptive cells, 

 because there is a very limited amount of evaporation from these plants, and the 

 quantity of water to be absorbed to replace what is lost is therefore small. The 

 same thing holds in the case of evergreen Conifers, in which, owing to the structure 



o o o 



of the stiff needles and to the peculiar formation of the wood, water is conducted 

 very slowly from the roots to the transpiring green organs. It has been ascertained 

 that they exhale from six to ten times less vapour than do ashes, birches, maples, 

 and other flat-leaved trees growing on the same ground. 



We shall presently return to the question of the substitution for absorptive cells 

 in many coniferous and angiospermic trees and in evergreen Daphnacem, Ericacea}, 

 Pyrolacea;, Epacrldea, &c, of the mycelium of fungi, and shall treat also of the 

 importance of the form of the absorptive cells, and of the roots which bear them, 

 in relation to the mechanism of striking root in the ground. 



