THE LEAF 
49 
The access of air to the leaf-cells for assimilation favours 
respiration also, and as far as the influence of function upon 
structure is concerned we may neglect respiration. 
The ordinary leaf described above has an upper or ventral and a 
lower or dorsal surface ; the internal structure differs on the two sur- 
faces, and such a leaf is termed dorsiventral. It reacts to gravity in a 
slight degree but is chiefly sensitive to light, and tends to take up during 
its growth a definite position with regard to the incident light (the fixed 
light position) ; in temperate climates this is usually at right angles to 
the brightest diffused light, i.e. approximately horizontal. Many leaves 
exhibit symmetrical internal structure, and stomata equally on either 
side, and place their edges to the light, eg. Iris, Narthecium, Eucalyp- 
tus, etc. ; such leaves are termed isobilateral ; they may develope in this 
position or attain it by twisting. Physiologically, th tphyllodes of Acacia 
(y. v .), &c. come under this head. In some species of Juncus, Allium, 
&c., the leaf is circular in section, with the tissues evenly distributed all 
round ; it stands erect, and is termed a centric leaf. The isobilateral 
and centric types of leaf offer less surface to radiation and hence are less 
liable to excessive transpiration by day or excessive cooling at night. 
Many leaves obtain these advantages by movement, however, and yet 
retain the chief value of dorsiventral structure, viz. the great amount of 
assimilation due to the extent of surface exposed to light. Most Legu- 
minosae execute sleep-movements with their leaves at night ; the blades 
(in various ways in different plants) move so as to place their edges in- 
stead of their surfaces upwards. Oxalis is also a good example and 
there are countless others (see Darwin’s Movements of Plants). In the 
tropics many leaves execute similar movements in the heat of the day, 
so that the light shall strike their surfaces obliquely, and thus not cause 
excessive decomposition of chlorophyll. The hanging of young leaves 
and shoots (see Chap. III.) is a similar phenomenon. We may also 
mention the ‘compass-plants’ Lactuca and Silphium. An exaggeration 
of the ordinary sleep-movements (which are regulated by the stimuli of 
light and temperature) is seen in Mimosa, Biophytum, Neptunia, &c., 
whose leaves take up the sleep position on being touched. Desmodium 
gyrans exhibits spontaneous movement without any apparent stimulus 
at all. The movements in all these cases take place by means of more 
or less swollen joints or pulvini. The pulvinus is chiefly composed of 
parenchymatous tissue, and the cells on one side gradually (or suddenly 
in Mimosa, etc.) lose their turgidity under the influence of the stimulus, 
while those on the other side retain theirs ; the result is a bending of the 
joint. When the flaccid cells regain their water the joint straightens 
once more. 
The external form of leaves presents extraordinary 
varieties in different groups of plants, even in nearly related 
forms, and we are at present almost ignorant of the factors 
determining it. The leaf commonly shows a distinction into 
the leaf-base or portion abutting on the stem, the leafstalk 
w. 
4 
