18 BULLETIN 183, U. S. DEPARTMENT OF AGRICULTURE. 



with it, and all later progress in conversion is made from it toward 

 the distal end of the grain. 



Tlie embryo is able to feed upon starch substituted for its normal 

 endosperm. Living embryos are able to grow when grafted on 

 endosperms which have been previously kiUed. 



The function of enzym secretion in germination is localized in the 

 epithehal layer of the scutellum. Embryos from which this layer 

 has been removed are unable to convert starch. 



SOURCE OF CYTATIC AND PROTEOLYTIC FERMENTS. 



The location of the secretion of diastase is not the only point in the 

 physiology of the gram upon which investigators have differed. Some 

 authorities who have placed this function in the scutellum have 

 ascribed the formation of cytase and of the proteolytic enzym to 

 other sources. 



With cytase, if the conclusions of the writers be correct, the ques- 

 tion is soon settled. If starch conversion starts next to the embryo 

 and moves toward the distal end of the grain, cytase must also be a 

 product of the epithelial layer. The breaking down of the cell walls 

 must always precede the action of diastalse. The diastase of digestion 

 seems to be unable to pass readily through unmodified cell walls, and 

 most certainly the cell walls are not affected much beyond the 

 depleted zone. These two enzyms, then, must of necessity not only 

 be associated in the changes which occur in the endosperm, but must 

 proceed from a common source. 



The proteolytic ferments would appear from analogy to present 

 no differences from the other two. Proteolytic action certainly does 

 not occur m any part of the endosperm not yet reached by cytase 

 and diastase. If the proteolytic ferments are secreted by the aleurone 

 layer they do not become operative until the cytase has moved 

 forward from the scutellum and has broken down the cell walls. 

 This lack of plausibility is not proof, but, on the other hand, there is 

 no reason for thinking that there is more than the single source of 

 enzymatic secretion. In one or two other grasses any other origin 

 than the scutellum is apparently impossible, because the aleurone 

 layer is digested almost as readily as the starch endosperm. In these 

 cases, at least, the secretion must be a product of the scutellum, and 

 in barley every indication short of absolute proof insists on the same 

 conclusion. In the opinion of the writers, the scuteUum as a feeding 

 organ is endowed with all the fmictions of digestion, being able to 

 utilize aU foods occurring in its natural storehouse, the endosperm. 



FUNCTION OF THE ALEURONE LAYER. 



These conclusions leave unexplained one very evident fact. The 

 aleurone layer is obviously a vital tissue. Its cells are in no way 

 similar to those of the starch endosperm beneath it. This marked 



