CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 705 



in the first instance, from that which fills the non-conducting 

 tissue of the wood (and the old wood or duramen, if present), and 

 ultimately by the root. It may be thought that the suction due 

 to transpiration would be incapable of maintaining the current; 

 but this difficulty is met by the consideration that the water is 

 held in position by the capillarity and the cellular structure of 

 the tracheidal tissue, and that the system of columns of water 

 and gas-bubbles does not move as a whole, since the latter cannot 

 pass the pit-membranes of tracheids. Moreover the force of 

 transpiratory suction is considerable, though it has not been 

 accurately measured. 



The Distribution of Organic Plastic Substances. These substances 

 may be generally stated to consist of organic substances of two 

 kinds, nitrogenous and non-nitrogenous, and these are distributed 

 through different channels. 



1. The nitrogenous substances travel, in plants or in parts of 

 plants which are not supplied with vascular tissue, in the form 

 of amides (see p. 707) by osmosis from cell to cell. But in 

 vascular plants it is known that they also travel in the sieve- 

 tissue from one member of the plant to another, in the form of 

 indi (fusible proteids. There is no evidence that the very slow 

 movement of the contents of the sieve-tubes is effected by any 

 special mechanism ; it appears to be simply induced by the de- 

 mand for these substances at any points, and it is doubtless 

 promoted by the swaying of the stem and branches. 



2. The non-nitrogenous substances travel through the plant in 

 the form of glucose and maltose (see p. 708), in solution; they 

 travel by diffusion from cell to cell, and more especially in the 

 elongated parenchyraatous cells, forming the conducting -sheath, 

 which, in the leaf, consists of mesophyll-cells closely investing the 

 vascular bundles, and, in the stem, belongs to the inner cortex. 

 This layer is ' not the endodermis, but lies externally to it : the 

 endodermis frequently contains starch-grains, and is sometimes 

 termed the starch-sheath, but it is rather a depository than a con- 

 ducting-tissue. 



The direction in which organic substances travel in the plant 

 seems to be determined simply by the demand for them. Just as 

 the water and the substances in solution absorbed by the roots 

 travel to the transpiring and assimilating organs, so the organic 

 substances produced in the assimilating organs travel in the 

 plant to those parts in which organic substance is either being 



