Chapter XV 



ORIGIN AND SIGNIFICANCE OF THE VACUOLES 



Aleurone grains : their formation. — Aleurone grains are found 

 in every seed. Their significance has been discussed for a long 

 time. These grains are to be found in large numbers in all cells of 

 the embryo and are especially voluminous in the cells of the coty- 

 ledons or of the endosperm. In the Gramineae they are localized 

 in the protein layer of the endosperm. 



Aleurone grains vary in structure. In many cases (RiciniLS, 

 Cucurbita Pepo) they are composed of an amorphous protein mass 

 enclosing a crystalloid of the same nature and one or several 

 spherical bodies called globoids, formed of phytin. Sometimes 

 twinned crystals of calcium oxalate are found in the protein mass 

 or in the globoids. In other cases the aleurone grains do not con- 

 tain crystalloids and enclose in their protein mass only numerous 

 globoids (Gramineae). Lastly, there are cases in which the aleurone 

 grains are composed entirely of amorphous protein (Legumes). 



Two opinions have been 

 formulated as to the 

 origin of aleurone grains. 

 Some authors considered 

 them to be plastids in 

 which were formed pro- 

 tein and the globoids. 

 Others contended that 

 they resulted from sol- 

 idification of the vacu- 

 oles during dehydration 

 of the seed. This latter opinion is now demonstrated to be true. 

 If, during the maturation of the seed, the vacuoles from any 

 part of the embryo or endosperm are examined using vital stains, 

 liquid vacuoles are found which are more or less large, according 

 to the type of cell and its state of development. These vacuoles 

 contain protein substances in solution and neutral red causes a 

 precipitation of these proteins within the vacuoles as deeply stained 

 bodies. In the course of development of the seed, the vacuoles be- 

 come much richer in protein. In the stages immediately preceding 

 maturation, i.e., at the time when dehydration takes place in the 

 seed, it is observed that the vacuoles sometimes have a tendency 

 to break up as they lose water, becoming smaller and smaller, and 

 less and less fluid. They stain deeply and homogeneously and are 

 now semi-fluid. They often at this moment become filamentous or 

 reticulate, analogous in form to those observed in meristems. 

 Later when the seed has reached maturity and passes into the 

 dormant stage, the semi-fluid vacuoles by more marked dehydra- 



FiG. 117. — Aleurone grains in seeds, a, h, Ricinus: 

 one crystalloid and 1-2 globoides. c, Oenanthe Phel- 

 landriuTn; sphaerocrystal of calcium oxalate in protein 

 ground substance. 



