Vegetable Physiology. 



395 



Avlth starch, since these two substances are very commonly 

 present together, and several hypotheses referring to the evolu- 

 tion of oxygen by green organs have been based upon supposi- 

 tions of convertibility of these substances one into the other. It 

 will be !nost convenient to postpone the consideration of these 

 questions until we have examined the characters and the develop- 

 mental history of the starch-granules themselves. 



Starch is at once one of the most important and most widely 

 diffused of all the substances produced in the interior of vege- 

 table cells. It is met with in all classes of plants except the 

 Fungi, at some period of their growth, and in almost all tissues 

 while in a young condition. It occurs, however, in greatest 

 abundance in certain special tissues or organs, which on this 

 account become of high value in an economical point of view. It 

 is almost superfluous to say that we refer here to seeds, such as 

 the cereal grains, &c. (fig. 11), tubers, like the potato and the 

 analogous root-like subterraneous stems of the arrowroot plants. 

 In the sago-plants it is accumulated in the parenchyma of the 

 trunk, and even in ordinary trees it is found in considerable 

 quantity in the winter season in the inner parts of the bark, the 

 outer part of the wood, and in the pith in the neighbourhood o[ 

 the winter-buds. 



Fiff. 11. 



Section of the outennost part of tlie wlieat-grain : a, the epidermis with a subjacent layer of cells 

 almost obliterated by pressure ; h, cells witli thiik and punctiittd w alls ; a and () are more or 

 less coloured, 6 giving the brown colour of bran ; c, a layer of clear colourless substance formed 

 by the obliteration of several layers of cells through pressure ; d, large thick wall-cells filled 

 with finely granular protoplasm, without starch-granules; e, cells, with delicate membranous 

 walls, filled with stiirch granules, forming the mass of the grain. 



