PALISADE TISSUE AND CHLOROPLASTS 109 



in the transverse section (cf. Fig. 51, Pa.). In sections 

 parallel to the surface of the leaf the pahsade cells appear 

 rounded (being cut transversely, Fig. 52, A), so that they have 

 the form of a number of closely packed cylinders placed side by 

 side and interspersed with regularly disposed vertical spaces (i.p.), 

 where the curved surfaces are not in contact. 



The numerous lenticular cliloroplasts ' form an almost con- 

 tinuous layer in the cytoplasm lining the vertical walls, a feature 

 well seen in 

 both trans- 

 verse and sur- 

 face sections 

 (Fig. SI, Pa.; 

 Fig- 52, A ; 

 see also Fig. 

 43). This 

 peripheral 

 position is 

 clearly favour- 

 able to the 

 rapid absorp- 

 tion of carbon 

 dioxide from 

 the adjacent 

 intercellular 

 spaces. More- 

 over the 

 chloroplasts, 

 as a result, present their edges to the light so that the chlorophyll 



1 For the general properties of chlorophyll, see F. & S., p. 13c. It is 

 now known that the chloroplasts of all plants contain two green (chloro- 

 phyll a and chlorophyll 6) and two yellow pigments (carotin and xantho- 

 phyll), the former being present in considerably greater quantity than 

 the latter. The chlorophjdls, which differ but slightly from each other, 

 are complex compounds of Carbon, Hj'drogen, Oxj-gen, Nitrogen, and 

 Magnesium, whilst the j-ellow pigments are of simpler composition ; 

 carotin has the formula C40H56, and xanthophyll the formula CjoHjoOo. 

 A rough separation of the green and yellow pigments may be effected by 

 shaking up an alcoholic extract with benzene and allowing the liquids to 

 settle ; the alcohol then contains the jrellow, the benzol floating above it 

 the green, pigments. 



Fig. 



52. — Sections parallel to the surfaces of a Fuchsia 

 leaf, cut respectively through the palisade laj^er 

 (A) and through the spongy parenchyma (B). In 

 each case the lower figure shows a small portion 

 on an enlarged scale, i.p., intercellular spaces. 



