392 CHLOROPHYLL AND LKiHT INTENSITY. 



rich in clilorophyll-graiiules than those of Hhaded plants, only they have developed 

 from their epidermal cells those structui-es which have been previously described 

 as covering hairs. These cellular structures, devoid of chlorophyll, cover over the 

 green tissue, and thus give to the leaf in question a grey or white colour. They 

 play the part of awnings and light-extinguishers, and when they are removed the 

 leaf appears just as green as one that has been plucked from the shade of the wood. 



Silky, velvety, and woolly coats may thus doubtless take on the function of 

 extinguishers. We meet, therefore, the same contrivances apparent!}' which already 

 on a previous occasion have been treated of, viz. when describing the protective 

 measures against excessive transpiration. Thus through these structures two birds 

 are killed with one stone. All contrivances which keep off too glaring sunbeams, 

 and thereby hinder the destruction of chlorophyll, at the same time diminish trans- 

 jjiration: and inasmuch as these contrivances perform two such important functions 

 for the life of plants, their wide distribution and great diversity is accounted for- 

 Suited to the conditions, adapted to the habitat and season of the year, and in 

 harmony with other developments, they change in a thousand ways, and thus 

 disjjlay a diversity which can scarcely be treated exhaustively. Besides the 

 •covering hairs which are placed above the gi'cen tissue, as a protection and shade 

 against too intense light, and a,t the same time against excessive transpiration, 

 obviously all the other contrivances previously described are to be taken into 

 account. The development of one or several layers of cells, filled with watery 

 cell-sap, above the tissue exposed to the sun's rays, the thickening of the cuticular 

 layers, the waxy and varnish-like coatings, the lime incrustations and salt 

 excretions, the diminution of the illuminated portion of the leaf-surface, the 

 formation of wrinkles, folds, pits, and grooves on the illumined surface of the 

 foliage — all these are able to interrupt and diminish the ra3's and to reduce their 

 intensity to the right degree. 



The number of the sjjecial contrivances which simply secure chlorophyll from 

 destruction by too glaring light, without at the same time protecting the green 

 tissue from excessive transpiration, must indeed be very small. First of all, we 

 may mention the dry thin-skinned scales which in many plants are inserted between 

 the green leaves. These are seen, for example, in species of the genus Paronychia, 

 which in masses have their habitat in sunny places, and produce silver-glittering 

 transparent scales, devoid of chlorophyll, close to that portion of the stem from 

 which the small green leaves originate. These scales, which are designated stipules, 

 and which, here, are usually as large, occasionally even larger, than the green leaves, 

 take up naturally such a position in the plants growing on shadeless hillocks that 

 the sun's rays first of all fall on them, and only reach the green leaflets in a 

 weakened state. 



Another arrangement, which indeed is able to restrict the destruction of the 

 chlorophyll by the sun's rays, without aflecting transpiration, consists in the 

 development of a blue or violet colouring-matter in those ceUs which compose the 

 superficial covering of the leaves and stem which is directly illuminated by the sun's 



