RELATIONS TO TRANSPIRATION 439 



features in the internal structure of foliage leaves and other transpiring 

 organs, which tend to encourage transpiration. It is the ventilating 

 system which most frequently exhibits hygrophilous adaptations of 

 this kind. For the sake of the resulting increase of transpiratory 

 activity, the intercellular spaces of a leaf may be enlarged to an extent 

 which would seem quite inexplicable from the point of view of photo- 

 synthesis and respiration. In the ordinary dorsiventral type of leaf, it 

 is the spongy parenchyma which performs the duties of a special tran- 

 spiring tissue, besides performing other functions as a part of the 

 photosynthetic system ; by far the largest proportion of the water- 

 vapour exhaled by a leaf is derived from the extremely well-ventilated 

 spongy mesophyll. The degree of development of this tissue, there- 

 fore, at once shows whether any given plant is distinctly hygrophilous 

 or xerophilous in character. In plants which are forced to reduce 

 their transpiration in order to avoid the danger of excessive loss of 

 water, the spongy parenchyma is comparatively small in amount and 

 scantily provided with intercellular spaces. This diminution of the 

 internal evaporating surface is often supplemented by a more or less 

 far-reaching reduction of the external surface exposed by the transpiring 

 organs. In the case of plants inhabiting damp, shady situations, or 

 more generally, wherever the nature of the environment tends to lower 

 the rate of transpiration, it is necessary that actively transpiring 

 tissues should be plentifully developed. As Areschoug has pointed 

 out, leaves in which the mesophyll is entirely made up of spongy 

 parenchyma, should probably be regarded in the main as extreme cases 

 of this hygrophilous adaptation. 212 



As a rule, adjustments of the quantitative development of the 

 ventilating system, such as those described above, suffice to effect the 

 requisite diminution or increase in the intensity of transpiration. A 

 peculiar arrangement of the foliar intercellular spaces which is found 

 in some Australian plants {Hakea, Restio, Kingia), and in Olca europea, 

 also seems to represent a xerophytic adaptation. In the majority of 

 leaves the palisade-cells abut directly against longitudinal air-passages, 

 which open into the ventilating spaces of the spongy parenchyma ; this 

 arrangement makes for comparatively active gas-exchange, because the 

 distances that have to be traversed by the diffusing gases are quite 

 short. In the above-mentioned xerophytes, these longitudinal passages 

 are replaced by tubular chambers, which encircle the cells in planes at 

 right angles to their long axis. The short connecting passages which 

 link up adjoining chambers, are all placed parallel to the surface of the 

 leaf, so that there is no direct communication between the chambers in 

 the radial direction. Tschirch 213 has pointed out, how greatly this unusual 

 arrangement of the intercellular spaces adds to the distance which 



