184 PART III. PHYSIOLOGY. [ 44 



at any rate for the most part, occupied by a system of short 

 columns of water with intervening gas-bubbles, the columns of 

 water being in communication by delicate films along the cell- 

 walls. If the whole of the tracheal tissue be in this state, it is 

 suggested that as water is withdrawn from the upper part of the 

 wood by the transpiration of the leaves, a current is set up, the 

 water travelling along the cell-walls, between them and the gas- 

 bubbles. But it may be that a continuous system of tracheids 

 completely filled with water is maintained, and that it is through 

 this that the current travels. The conducting-tissue is supplied 

 with water, 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 difficiilty 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, thoxigh it has not been 

 accurately measured. 



The Distribution of Organic Plastic Substances. These sub- 

 stances may be generally stated to consist of organic substances of 

 two kinds, nitrogenous and non-nitrogenous, and these are dis- 

 tributed 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. 186) 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 

 indiffusible 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 point, 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. 187), in solution ; they 

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

 elongated parenchymatous cells, forming the conducting-sheath, 

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

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



