MECHANICS EFFECTS OF INSOLATION. 197 



Effects of Insolation. It can be determined without difficulty 

 that the leaves of the Beech have grown especially thick in 

 sunny places, and are so much the thinner in proportion to 

 shade. This increase in thickness, as microscopical investigation 

 shows, affects the palisade parenchyma, which can become 

 very considerably elongated and multilamellar. The palisade 

 parenchyma is indeed a tissue specially adapted for strong light- 

 intensity, while the spongy parenchyma is suited for feebler 

 light. In the palisade cells we see the chlorophyll grains 

 only in profile, i.e., distributed over the elongated side- walls, 

 and therefore, in proportion to the intensity of the illumina- 

 tion, only projecting more or less slightly into the cavity of 

 the cell. In the spongy parenchyma, on the other hand, the 

 chlorophyll grains, according to intensity of illumination, show 

 either surface or profile arrangement, i.e., lie parallel to or perpen- 

 dicular to the upper surface of the leaf. The chlorophyll grains 

 in the palisade layer are first affected by the sun's rays ; while 

 the spongy parenchyma only receives light already weakened 

 by absorption in the palisade cells. This disadvantage is partially 

 compensated for by the surface distribution possible in the 

 spongy parenchyma. If, however, the intensity of the illumina- 

 tion is too great for the spongy parenchyma, its chlorophyll 

 grains assume the arrangement in profile. In Beech leaves 

 which are developed in the most intense sunlight, almost the 

 whole green tissue is formed of palisade parenchyma ; while the 

 leaves, somewhere about a third their thickness, which have grown 

 in deep shade, have hardly anything but spongy parenchyma. 



Physiological Anatomy of the Leaf. We will introduce a few 

 more physiological conceptions into these morphological studies, 

 and test their accuracy by means of microscopical structure. 



In certain coloured chromatophores, and, so far as the more 

 highly-organised plants are concerned, exclusively in the green 

 chlorophyll bodies, the assimilation of carbonic acid takes place ; 

 that is, these coloured plastids alone are capable, in light of 

 sufficient intensity, of decomposing carbonic acid gas and water, 

 and out of them forming compounds rich in carbon. This 

 process takes place to by far the largest extent in the palisade 

 cells : and from a physiological point of view these cells can be 

 described as, in the highest degree, the assimilating or nourish- 

 ing cells, The palisade cells are, as we have already seen, more 

 or less completely separated from one another laterally, and come 



