592 LECTURE XXI. 



This is proved by the fact that in leaves grown in feeble 

 light the spongy tissue is well developed at the expense 

 of the pallisade-tissue. But when a leaf is exposed to bright 

 light, not only the constructive metabolic processes, but the 

 decomposition of the chlorophyll has to be taken into con- 

 sideration. The pallisade-tissue is to be regarded as a means 

 of diminishing the intensity of the light as it penetrates into 

 the leaf, and so of protecting, as it were, the chlorophyll 

 of the subjacent spongy tissue. These considerations afford 

 the explanation of the principal difference in structure be- 

 tween the horizontal and vertical leaf. It may be added 

 that the different distribution of the stomata in the two cases 

 is correlated with the differences of internal structure. 



If it be enquired why the cells of the mesophyll of a leaf- 

 surface exposed to bright light should assume the elongated 

 form characteristic of the pallisade-tissue, and why these 

 cells should be so placed in the leaf that their long axes 

 should be, as they are, parallel to the direction of the inci- 

 dent rays, the answer is again, that it is for the protection 

 of the chlorophyll. It was mentioned in previous lectures 

 (p. 299, see Fig. 36) that under the influence of changes in 

 the intensity of the incident light, the position of the chloro- 

 phyll-corpuscles in the cells is changed. The effect of 

 bright light is to induce light-apostrophe, that is, a position of 

 the corpuscles in which they present, not their flat surfaces, 

 but their edges, to the incident rays. The assumption of 

 this position can, obviously, be most readily carried out when 

 the form of the cell is a hollow cylinder placed with its long 

 axis parallel to the direction of the incident rays, conditions 

 which are exactly fulfilled by the cells of the pallisade- 

 parenchyma. 



But to return from this digression. In a previous lecture 

 we have spoken of leaves which when fully exposed to light, 

 take up a horizontal position, as being diaheliotropic, and 

 we may speak of those which, under similar circumstances, 

 take up a position which is more or less inclined to the 

 horizontal, as being paraheliotropic. The significance of the 

 fixed light position, whatever it may be, is that it is the 



