VARIOUS MOVEMENTS IN PLANTS 163 



way in which the leaves of many plants expose their upper 

 surfaces as fully as may be to the light, partly, as we may 

 see, in terms of their spiral origin upon the stem, though 

 with definite individual and collective adjustments, and 

 of various kinds. Thus a rosette-plant, like Dandelion, 

 may have its leaves all practically on the ground-level ; 

 but where there is some little stem, the lower leaves may 

 have longer stalks, so as not to be shaded by those 

 above. In most herbs and shrubs, when we look at 

 their leafage from the mid-day sun's point of view, 

 we may often admire the co-adjustment by which leaves 

 avoid shading each other, fitting themselves into a pattern, 

 often recalling those of wall-papers, or stuffs adorned 

 with decorative plant-designs. For this there is manifestly 

 some adjustment : some movement has taken place to 

 turn this and that leaf into a better position for light than 

 that of their simple and regular development upon the 

 stem. This further adaptation is effected through the 

 varying growth and movement, not only adjusting the 

 level of the leaf, but also, it may be, twisting it ; and we 

 seek to note how this is done. It is often effected by 

 the more or less enlarged and swollen-looking, because 

 turgid, leaf -base, the ' pulvinus/ which is conspicuous 

 in many plants, and highly sensitive in Mimosa. 



There are many other adaptations for that quest of 

 light on which the whole green world depends, and to 

 utilise which is the essential photo-synthetic activity of the 

 leaf, on which all animal life also depends, directly or 

 indirectly. Here, for instance, is the great practical value 

 of the stem and copious branches of tall herbs and shrubs, 

 and above all of trees ; for by the help of these they more 

 and more increase their available leaf area for light exposure, 

 so that a single tree of moderate magnitude is enabled by 

 the vast collective surface of its leaves to absorb a very 

 large amount of light. 



The light-quest of the plant-world appears in yet more 

 striking ways, so that each organ may find its place in 



