TRANSPIRATION AND ABSORPTION OF WATER 231 



temperature is low, and partly because the stomata are closed. Hence 

 the roots are able to absorb sufficient water to balance the loss by transpira- 

 tion, as was first noticed by Hales, and confirmed by Duhamel by observa- 

 tions upon a branch of an evergreen oak grafted on a deciduous one 1 . 

 If during winter branches of a plant growing in the open air are led into 

 a greenhouse they develop transpiring leaves, and this affords an additional 

 proof that in spite of the low temperature a large quantity of water may be 

 absorbed by the roots and transported through the stem 2 . Other plants, 

 however, are unable to extract sufficient water from a frozen soil, so that 

 they become flaccid and are ultimately injured when the external conditions 

 are such that the subaerial parts transpire actively. Hence Kihlmann 3 is 

 probably correct in his interpretation of the meaning of the protective 

 modifications against excessive transpiration which are shown by many 

 northern plants, although they grow in moist soil or swampy habitats. By 

 these means the plant protects itself against fatal or injurious loss of water 

 when the temperature of the soil is low and absorption is difficult. 



In the normal progress of development the conductivity of a tracheal 

 element (Sect. 35) gradually diminishes as time goes on, as does the 

 absorptive power of any particular region of a root. Similarly the 

 permeability of a cut surface of a stem gradually diminishes when kept in 

 water, so that after a few days the leaves begin to droop. This fact 

 was well known to Hales 4 ; it is due to the blocking of the opened tracheae 

 with mucilage, bacterial zoogloeae, &c., while in addition changes may 

 take place in the conducting tissue-elements near the cut surface, rendering 

 them less efficient for water-transport. 



When a stem is cut across, air is drawn into the opened tracheae and 

 tracheides owing to their internal negative pressure, and hence the absorption 

 of water is rendered more difficult. In herbaceous plants the lessened rate 

 at which water is then absorbed is sufficient to cause pronounced flaccidity, 

 even though the cut stem is immediately placed in water. If the section 

 is made under water the latter is drawn into the tracheae instead of air, 



1 Hales, Statics, 1748, p. 29; Duhamel, De 1'exploitation des bois, 1764, T. I, p. 337. 



2 Experiments of this kind were performed by Duhamel (Naturgesch. d. Eaume, 1765, Bd. n, 

 p. 219) and Knight (Treviranus, Beitrage zur Pflnnzenphysiol., 1811, p. 120) on vines, by Mnstel 

 (cf. de Candolle, Physiol. of Plants, T. I, p. 426) upon other woody plants. 



3 Kihlmann, Pflanzenbiol. Studien aus Russisch-Lappland, 1890, p. 104 ; Stenstiom, Flora, 1895, 

 p. 153. [In a swampy soil the deficiency of oxygen may render absorption more difficult, and 

 Nilsson even concludes that the xerophytic habit may be directly due to the poverty of the soil 

 (cf. Bot. Centralbl., Bd. LXXVI, p. 9), for in richer siil the xerophytic character may disappear. It is 

 also possible that, as in the case of mangrove vegetation, the xerophytic adaptation may aid in 

 avoiding an over-accumulation of injurious saline and other substances present in the soil and con- 

 veyed to the leaves by the transpiration current. Cf. also Rosenberg, (Jber die Transpiration der 

 Halophyten, Konigl. Vetenskaps-Akad. Forhandlingar, 1897, No. 9, p. 531.] 



* Hales, Statics, 1748, p. 18. Cf. also Unger, Studien z. Kenntniss d. Saftlaufes in d. Pflanzen, 

 1864, p. 3 (Sep.-abdr. aus Sitzungsb. d. Wien. Akad., Bd. L). 



