LECTURE XVI. 



EXCRETION OF WATER IN THE LIQUID STATE. 



Although the phenomena to be treated of here are far less important for the 

 life and maintenance of the plant than transpiration — i. e. the excretion of aqueous 

 vapour by land-plants — it is nevertheless worth while to examine more closely 

 the excretion of liquid water which takes place in plants under very various 

 circumstances ; especially since it aifords us a deeper insight into relations of 

 structure and of mechanical arrangements in the plant. From this point of view 

 the excretions of water termed ' bleeding ' and ' weeping ' particularly excited the 

 interest of the older vegetable physiologists at the end of the seventeenth and 

 beginning of the eighteenth centuries. 



By the terms Bleeding and Weeping are designated those excretions of water 

 which occur under certain circumstances in consequence of injuries, generally such 

 that water is exuded in greater or less quantity from fresh transverse sections 

 of the wood. Two essentially different cases have to be distinguished, however, 

 viz., first, the case in which water is exuded from the wood-body in consequence 

 of a rise of temperature, and in which, above all, the activity of the roots does 

 not take any part; and secondly, the case where, from wounds in the wood, 

 relatively large quantities of water are exuded during long periods, which did 

 not previously exist in the wood, but must first have been absorbed by the roots. 

 The first phenomenon, which I shall name the bleeding of wood in winter, can 

 be' produced just as wpll with isolated cut-oif pieces of wood as with rooted 

 woody plants ; while the other phenomenon, which I distinguish as the weeping of 

 the root-stock, only occurs in vigorous, rooted, living plants, although similar exuda- 

 tions of water, but very much less in quantity, may also occur under certain 

 circumstances with separate portions. 



The bleeding of wood in winter ^ is observed in its most typical form if, on 

 a cold but frostless winter day, a portion of a branch of a tree (Rhamnus, 

 Hazel, Pine, Walnut, Birch, &c.), about 25-50 centimetres long and 2-5 

 centimetres thick, is cut off, and the two cut ends smoothed with a sharp knife. 

 Outside in the cold air the smooth sections appear relatively dry, and no 

 liquid water is to be seen, even at the lower section, when the piece of branch 



* Cp. my treatise, ' Quellungserscheinungen an Hohern' (Bot. Zeit. i860, p. 253), where the 

 older literature is collected also. The explanation of the phenomena there described by me is found 

 in my treatise, ' Ober die Porositat des Holzes ' (Arb. des bot. Inst, in Wzbg. B. II, p. 291). 



