218 VEGETABLE PHYSIOLOGY 



of starch in the neighbourhood of a growing region. 

 Grains of starch are of frequent occurrence in different 

 parts of the bast, and particularly in the bundle-sheaths of 

 certain regions. The explanation of their appearance 

 there is simple ; they are generally indications of such an 

 interference with the supply and the demand as we have 

 described. A checking of the demand by a cessation of 

 the vigour of growth or nutrition is attended by an over- 

 accumulation of the sugar, which is speedily changed into 

 a storage form instead of being removed by the slow pro- 

 cess of diffusion. 



The transport of proteins follows the same course ; the 

 amino- or amido -acids are the travelling forms, and are 

 conducted by the same forces to the growing points, or to re- 

 servoirs where accumulation of proteins takes place. Their 

 deposition in storage forms along the pathway can also be 

 detected, though these are not so widespread as those of 

 carbohydrates. They can be observed generally in the sieve- 

 tubes of the bast, which contain a curious modification of 

 protoplasm in which protein as such is present. It was 

 formerly held that the sieve-tubes conduct protein as such 

 along the vascular bundles. Though there is not a very 

 great improbability that such bodies may pass from cell 

 to cell of the sieve-tube, on account of the protoplasmic or 

 quasi-protoplasmic threads which extend throughout the 

 openings of the sieve-plates, yet this method of transport 

 must be necessarily very slow and subject to much 

 hindrance. It seems more probable that the proteins in 

 these vessels are constructed there from the amino-acids 

 which reach them, and are to be regarded as temporary 

 stores, like the starch grains already alluded to as being 

 formed in different parts of the translocatory tract. 



We have spoken of the bast as forming the pathway of 

 the translocation of nutritive material or of the different 

 food-stuffs which have been manufactured. The process by 

 which they travel, we have seen, is mainly one of diffusion 

 through the cell-walls, the latter being saturated with the 



