19 



is changec^, and so forth. Some histological variations of this 

 kind are to he discussed in the follov/ing. 



1. The struct-ure of the leaf tissue is dependent, to a 

 high degree, on the action of external factors, the number of 

 cell-layers- which form the mesophyll and epidermal tissues varies 

 with the life conditions of the plant. The difference between 

 (24) the leaves of many plants exposed to the raj;^s of light and those 

 grown in shade, verified hy Stahl and other authors, is well- 

 known-*-. In the mesophyll of the sun leaves of Fagus sllvatiofe 

 (compare fig. 5 b.) Six to eight or even more cell layers lie 

 one above the other,- in the shade leaves only three (fig. 5c). 

 With the medium supply of light, the development of the meso- 

 phyll ^(ieps the mean, (fig. 5a). This arrestment, due to in- 

 sufficient exposure to light, is undergone also by the meSophyll 

 of other plants, further hy the epidermis of those plants, which 

 normally develop a many-layered upper covering. Figure 8 demon- 

 strates the difference betv/een the epidermis of a sun leaf, and 

 a shade leaf of FiotLg, stipulata ; the 'cross divisions are lack- 

 ing in the leaf kept in shade. We will return later to other 

 differences between shade and sun leaves. 



The characteristic development of shade and sun leaves is 

 determined less "by light itself than by transpiration, xvhich 

 proceeds in the latter much more actively than in the former. 

 On this account, leaves with the scantily developed mesophyll 

 of shade leaves may be produced in damp air with an abundant 

 supply of light '^. 



Griffon verified the reduction of the number of layers 

 in Canna, Chrysanthemum and others, in his comparison of the 

 pale green varieties v/ith those normally green. 



■^ Compare especially?- Stahl, Ueber den Einfluss der Lich- 

 tintensitat auf Struktur und Anordnung des Assimilationspar- 

 enchyms. Bot. Zeitung, 1880, Bd, XXXVIII, p. 868. Ueber den 

 Einfluss des sonnigen und schattigen Standortes auf die Aus- 

 bildung der Laubblatter. Jenaische Zeitschr. f. Naturwissen- 

 schaften, 1883, Bd. XVI. Further, Pick, Ueber den Einfluss 

 des Lichtes auf die Gestalt und Orientierung der Zellem des 

 Assimilationsgewebes, Botan, Centrabl, 1892, Bd. XI, p. 400; 

 Haberlandt, Physiol. Pflanzenanatomie , 2. Aufl. 1896, p. 252. 

 (More literature references therein). Compare also the lit- 

 erature quoted on the following pages. 



^ Compare the experiments of Lothelier, Rech. sur les pi, 

 a piquants. Rev. gen. de Bot., 1893, T. V. p. 480; further 

 Vesque, Sur les causes et sur les limites des variations de 

 structure des vegetaux. Ann, Agron., 1884, T, IX and X. 

 Vesque et Viet. De I'infl. due milieu sur la struct, anat, 

 des. vegetaux. Ann. Sc. Nat. Bot., VI. serie , T. XIII, 1881, 

 p. 167. 



^ L 'assimilation chlorophyllienne et la coloration, Ann. 

 Sc. Nat, Bot,, VIII, serie, T. X. 1889, p. 1. 



