Power to Adjust Parts to Surroundings 233 



of these vertical light-positions is believed to consist in a pro- 

 tection given to the living substance of leaves against the full 

 exposure to a brightness too intense for their good; for we know 

 on the one hand, that too bright a light does chemical damage to 

 protoplasm, even when partially screened 

 by the chlorophyll, while on the other hand, 

 leaves can make use of only a moderately 

 strong light, the extra brightness being 

 wasted upon them. It is this last-mentioned 

 circumstance, by the way, which explains a 

 problem that sooner or later will puzzle the 

 reader, viz., why all the vegetation in the 

 northern hemisphere does not have a turn 

 towards the south where the sun is. This 

 is no doubt because the diffused light falling 

 on the plants from the north is quite as 

 strong as they can use; and hence they have 

 no object, so to speak, in turning to the side 

 of the sun. 



There remains one other phase of photot- 

 ropism in leaves which must here be consid- Flo . 78 ._^ Uuitof ^^ 

 ered, and that is their lateral shiftings out tus > showing the position 



assumed in the bright 



from beneath one another s shade, a move- sun by the leaflets which 

 ment chiefly accomplished by twisting and ^f "co^Trom"* 

 lengthening of the petioles. The result is paper by w. p. Wilson.) 

 often to bring them, especially in spread-out plants like the 

 vines, into a one-planed pattern where no leaf is overlapped 

 by another, an arrangement commonly known as a leaf-mosaic 

 (figure 79) ; and there are even some botanists who believe that 

 the angular shapes of such leaves (e. g. in the English Ivy) are 

 partly determined by the advantage of interlocking to use all 

 the space. 



Such lateral shiftings imply that the whole upper surface of 

 the leaf is equally receptive to the light stimulus; and a very 



