54 



BOTANT. 



tionally in some plants and uniformly in others) two or more 

 nuclei occur in each grain ; by growth such grains become 

 compound and may finally separate into as many parts as 

 there are nuclei. 



69. — The molecular structure of the starch grain has been 

 determined to be similar to that of plant-cellulose. It is re- 

 garded as composed of molecules, each of which is surrounded 

 by a watery layer of greater or less thickness. Growth takes 

 place by the intercalation of new molecules between the pre- 

 viously formed ones — in other words, by intussusception, 



exactly as in the case of 

 ^ the cell-wall. During ihe 

 formation of the grain, in 

 L C77/@ certain portions of it the 

 IkWp® watery layers surrounding 

 the molecules become 

 thicker. When seen by 

 transmitted light such 

 more watery parts appear 

 darker than those which 

 are less watery, and an ex- 

 >|© amination shows that they 

 surround the nucleus on 

 all sides in a concentric 

 manner. In this way -the 

 starch grain comes to be 

 made up of alternating 

 layers of more and less 

 watery substai:ee. Every watery layer is thus between two 

 layers which contain less water, and so every less watery one 

 lies between two more watery ones. As an increase in the 

 amount of water in any portion of the starch grain de- 

 creases the density of that portion, the layers just described 

 may be distinguished as of greater density when having 

 less water and of less density when having more water. 



Kg. 44, . , , 



(Fiffum sativum). St, starch grains with nucleus 

 and concentric striie ; a, granules of aleurone ; 

 i, 2, intercellular spaces. X 800. — After Sachs. 



-Cella from the cotyledon of the pei 

 " " ■ iclei: 



should be abandoned, as it was originally given under the mistaken 

 notion that it was the point of attachment of the starch grain,while 

 growing. 



