88 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



Nevertheless, the leaves which have the greatest surface have not 

 always the greatest thickness and weight, nor does their tissue-differen- 

 tiation and power of photosynthesis by any means always reach the 

 maximum possible 1 . 



The influence of light in favouring the development of palisade 

 parenchyma is a factor of great biological utility, and in plants growing 

 in deep shade this layer may be developed feebly or not at all 2 (Fig. 19). 

 Again, in the absence of light the thickening of the cell-walls is less pro- 

 nounced, and hence etiolated shoots and leaves are less rigid than normal 3 . 

 The laying of cereals is in fact usually due to the basal portions of the haulm 

 being so much shaded by close planting that they become etiolated, and 

 do not acquire sufficient strength to support the ripened ears. Owing to 

 the lessened rigidity of the cell-walls, the tissue-strains are less pronounced 

 in etiolated stems than in those exposed to light, and indeed they may, in 

 some cases, be so altered by the relatively more active growth of the cortex, 

 that the latter instead of being stretched is compressed longitudinally 4 . 



Etiolation was recognized by Ray, and later by Bonnet, as being due to 

 the absence or deficiency of light 5 . Senebier and de Candolle pointed out 

 that specific differences exist in this respect between different plants, and 

 between the different parts of the same plant 6 . Our knowledge was further 

 extended by Sachs, G. Kraus, and subsequent authors. Senebier observed that 



des sonnigen u. schattigen Standorts, &c., 1883, p. 29; Dufour, Ann. d. sci. nat., 1887, 8 e ser., T. v, 

 p. 407; Wiesner, Sitzungsb. d. Wien. Akad., 1893, Bd. en, I, p. 321; Teodoresco, Rev. gen. 

 d. Bot., 1899, T. xi, p. 433. The stages in the sizes of the leaves are well shown by ivy plants 

 growing in the clefts of rocks. 



1 Ge"neau de Lamarliere, Rev. gen. d. Bot., 1892, T. IV, p. 481. 



3 Haberlandt, Physiol. Anat., 1896, 2. Aufl., pp. 253, 260; Teodoresco, Ann. d. sci. nat., 



1899, 8 e ser., T. X, p. 433, and Rev. gen., I.e. If the light is too intense the differentiation of the 

 tissues is again decreased (Bonnier, Rev. gen. d. Bot., 1895, T. vn, p. 412). On Marchantia cf. 

 Kammerling, Flora, 1897, Erg.-bd., p. 53. On subterranean leaves cf. Thomas, Rev. gen. d. Bot., 



1900, T. xil, p. 394. 



3 Kraus, Jahrb. f. wiss. Bot., 1869-70, Bd. vil, p. 209; Batalin, Bull. d. 1'Acad. de 

 St.-Petersbourg, 1871, T. xv, p. 21; Rauwenhoff, Ann. d. sci. nat., 1878, 6 e ser., T. v, p. 267. 

 The cells in elongated etiolated stems are usually somewhat longer than in normal internodes, but 

 are smaller than usual in the small leaves formed in darkness (G. Kraus, 1. c., pp. 234, 259 ; 

 Rauwenhoff, I.e., pp. 285, 310; Batalin, Bot. Ztg., 1871, p. 676; Prantl, Arbeit, d. Wiirzburger 

 Instituts, 1873, Bd. I, p. 384). Hales (Statics, 1748, p. 188) was the first to observe the greater 

 softness of etiolated plants, while Knight (Phil. Trans., 1801, II, p. 348) showed that roots freed from 

 the soil form firmer wood. Cf. also Wiesner, Bot Ztg., 1884, P- 675. On the influence of etiolation 

 on hairs cf. Schober, Bot. Centralbl., 1886, Bd. xxvin, p. 39; on cystoliths, Kohl, Kalksalze u. 

 Kieselsaure i. d. Pflanze, 1889, p. 139. 



* Sachs, Bot. Ztg., 1863, Eeilage, p. 13; Kraus, I.e., pp. 240, 250. Cf. also Rauwenhoff, 

 1. c., p. 295. 



5 Ray, Historia plantarum, 1686, Vol. I, p. 15; Bonnet, Recherches sur les feuilles, 1762, 

 122-89. 



6 Senebier, Phys.-chem. Abhandl., 1785, 2. Thl., pp. 52, 103, no; de Candolle, Physiolog. 

 ve"ge"tale, 1832, Bd. Ill, p. 1078. De Candolle erroneously supposed that only green parts can 

 become etiolated. 



