360 THE FOOD OF PLANTS 



secure more even distribution of the incident rays, and in the lower palisade 

 layers the light may be concentrated as it is in the biconvex protonema cells 

 of Schizostega osmundacca^ in which the chloroplastids collect at the points 

 on which the incident light is focussed l . 



These requirements are fulfilled in a dorsiventral leaf by the deve- 

 lopment of palisade parenchyma on the directly illuminated surface 2 , while 

 a certain quantity of light may penetrate to the spongy mesophyll in 

 which the chlorophyll is less abundant, and which also receives light 

 reflected from beneath. The large intercellular spaces of the spongy 

 mesophyll renders possible the rapid transference of carbon dioxide to the 

 palisade parenchyma, and when stomata are present only on the under 

 surface of the leaf practically the whole of the carbon dioxide absorbed 

 by the palisade layers passes through the intercellular spaces of the meso- 

 phyll (Sect. 31). According to the structure of the leaf and its specific 

 properties photosynthesis may be more or less active when the under surface 

 is turned upwards, or when the leaf is illuminated from beneath 3 . 



It is in correspondence with the general and purposeful self-regulatory 

 power that organs whose functional activity depends upon illumination 

 should be markedly affected by the intensity of the light under which they 

 develop. Thus in darkness most leaves do not turn green nor is their 

 normal shape or structure attained, while the maximal size is usually 

 reached in light of moderate intensity, and it is only under strong illumina- 

 tion that the greatest possible thickness and maximal tissue differentiation 

 are attained. Leaves grown in the shade may be without any definite 

 palisade layer, while two to three layers may be present in leaves which 

 have developed in exposed situations 4 . Hence when grown in moderately 

 bright light leaves very often become deeper green in colour, and contain 

 more chloroplastids to the same area than when developed in shady 

 situations 5 . 



The influence of light upon growth, movement, &c., will be discussed 

 later. It may, however, be mentioned that changes of illumination do 



1 Noll, Arb. d. Bot. Inst. in Wiirzburg, 1888, Bd. Ill, p. 477. 



a Cf. Haberlandt, Physiol. Anat., 1896, a. Anfl., p. 226, where other literature is quoted. 

 Jbnsson, Zur Kenntniss d. anat. Baues d. Blattes, 1896. 



3 See Kreusler, Landw. Jahrb., 1890, Bd. xix, p. 662 ; Meissner, Bot. Centralbl., 1894, Bd. LX, 

 p. 206. 



* Chief literature : Stahl, Uber d. Einfl. d sonnigen u. schattigen Standorts auf Laubblatter, 

 1883; Heinricber, Jahrb. f. wiss. Bot., 1884, Bd. xv, p. 556; Johow, ibid., p. 284; Grosglik, 

 Bot. Centralbl., 1884, Bd. XX, p. 374 ; Haberlandt, Ber. d. Bot. Ges., 1886, p. 206 ; Dufour, Ann. d. 

 sci. nat., 1887, vii. se"r., T. V, p. 311 ; Lamarliere, Rev. ge"n. d. Bot., 1892, T. iv, p. 481 ; Ssurosh, 

 Bot. Jahresb., 1892, p. 95. On marine algae, cf. Berthold, Jahrb. f. wiss. Bot., 1882, Bd. XIII, 

 p. 690. 



fl Lamarliere, I.e., p. 492; Bonnier, Ann. d. sci. nat., 1894, vii. ser., T. xx, p. 337 (Alpine 

 plants). 



