TRANSPIRATION AND ABSORPTION OF WATER 233 



ultimately induces more rapid absorption, until a new condition of equilibrium is 

 reached which corresponds to the changed conditions, so that the amounts absorbed 

 and exhaled again correspond. The effects of changes of temperature are compli- 

 cated by the changes of volume which they cause. Thus when the plant is warmed 

 water may be driven out of it by the expansion of the air enclosed in the tracheae. 

 This takes place when fairly moist wood is warmed, so that the specific gravity 

 of the wood is lessened and it may eventually float in warm water, though it 

 sinks when first immersed 1 . 



The amount of water which trees contain is liable to variation, just as is the 

 case in herbaceous plants. Th. Hartig, and more especially R. Hartig, have 

 shown that, as might be expected, the wood usually contains more water in 

 summer than in winter, the minimum commonly occurring in autumn, the 

 maximum in spring. The data obtained by Gele/.now and Duhamel agree with 

 these results '-'. In the latter case the accumulation of water causes a considerable 

 internal pressure to be developed, while when the leaves expand the amount 

 of water which the stem contains undergoes a marked diminution. In deciduous 

 trees the spring maximum is more marked than in evergreens, as might naturally 

 be expected, since the latter begin to transpire actively immediately the external 

 conditions become favourable. It is hardly surprising that the curves obtained 

 for different plants should not be precisely similar, and that variations may be 

 shown in different years. During a prolonged rainy period the wood will un- 

 doubtedly contain more than the normal amount of water, while under special 

 conditions a daily variation may become perceptible. 



The energy with which water is absorbed is not necessarily directly dependent 

 upon the amount of water which the plant contains. We need not therefore 

 give a detailed account of the variations in the amount of water observed in 

 the upper and lower portions of the trunks of trees, &c. (see Th. Hartig, 1. c.). 

 These variations occur for the most part in the alburnum, and only to a slight degree 

 in the duramen. The latter normally contains but little water, and the amount 

 alters only slightly even when transpiration is very active (Sect. 35). Hartig found 

 that at a point 10 metres from the ground, the amount of water present in the 

 stem of a beech varied from 41 to 85 per cent, (the total area of cell-spaces 

 being 100). 



Rauwenhoff, Archives neerlandaises, 1868, T. in, p. 318; J. Bonssingault, Agion., Chim. agr., &c., 

 1878, Vol. vi, p. 301 ; Vesque, Kohl, Eberdt, in the literature quoted on p. 212. 



1 This phenomenon was first observed by Dalibard, Duhamel, &c., and more closely studied by 

 Sachs (Bot. Zeitung, 1860, p. 253 ; Arb. d. Bot. Inst. in \Viirzburg, 1879, Bd. n, p. 317). A correct 

 explanation was first given by Hofmeister (Flora, 1861, p. 101). 



2 Th. Hartig, Bot. Zeitung, 1868, p. 17 ; 1858, p. 329 ; R. Hartig, Lehrb. d. Anat. u. Physiol., 

 1891, p. 202; Unters. aus d. forstbot. Inst. zu Munchen, 1882, II, p. 20; 1883, ill, p. 47; Holz d. 

 Nadelwaldbaume, 1885, p. 95 ; Geleznow, Melang. biologiques tires du Bulletin, d. 1'Academie. 

 d. St.-Petersbourg, 1872, T. ix, p. 667 ; Ann. d. sci. nat, 1876, vi. ser., T. in, p. 344; Duhamel, De 

 1'exploitation des bois, 1764, p. 476. 



