TISSUES AND SIMPLE ORGANS. 103 



we shall here emphasize the fact that the following tissues are 

 especially adapted to conduct starch and other food-substances 

 which are free from nitrogen : the vascular bundle-sheath of the 

 leaf -blade and the parenchyma immediately about the larger bundles, 

 the parenchyma of the petioles and stern-organs, inclusive of the 

 wood-parenchyma and the medullary rays. The l ' path ' ' broadens 

 continually, similar to the path of blood -circulation in man. 



y. Conduction of Water. 



The history of our science has undergone great oscillations in 

 regard to the movements of water in the plant-body. By c ' water ' ' 

 we mean the liquids which, as is known, always contain mineral 

 salts and other substances in solution, and which are taken from 

 the soil by the roots. 



It is generally agreed that the current of water passes upward 

 in the woody tissue of dicotyledons and conifers, especially in the 

 younger annual rings (splint-wood). Let us further consider cir- 

 culation in dicotyledons and conifers. 



In agreement with many authors, I find it impossible to accept 

 SACHS' theory of ' ' imbibition. ' ' The future history of botany 

 will no doubt show that it was the reputation of the originator of 

 this hypothesis that made its acceptance possible for so long a time. 

 According to this hypothesis, we are to assume the following: 

 water does not pass through the , cell-cavities (lumina), but through 

 the cell- wall substance. The living woody cell- wall contains water 

 obtained by imbibition, and so long as the cell-wall is not dry this 

 water which has been imbibed is very readily displaced. Unligni- 

 fied (not woody) cell- walls and woody cell- walls which have once 

 become dried do not possess this property. Sachs further empha- 

 sizes the fact that the force with which water is retained in the cells 

 which are capable of imbibition is so great that it makes no dif- 

 ference whether the imbibing cell- body lies ten or one hundred 

 metres above the water-absorbing roots, just as in the case of the 

 saline ocean-water it makes no difference whether the salt-molecule 

 in solution floats one hundred or one thousand metres above the 

 bottom of the ocean." If from such a system of water- soaked 

 cells water is removed at one end by evaporation (in the leaves), 



1 Vorlesungen, page 290. 



