GERMINATION OF THE GRAIN 31 



soluble nitrogenous compounds, which are absorbed by the growing 

 embryo with the rest of the soluble plastic materials. 



An examination of germinating wheat from day to day shows that the 

 dissolution of the reserves begins in the amyliferous parenchyma behind 

 the upper part of the embryo, and spreads gradually towards the apex of 

 the grain beneath the aleuron layer on the dorsal side, extending also 

 round the flanks towards the ventral furrow. 



Rapid action of the enzymes is promoted by the early hydrolysis of the 

 cell membranes of the endosperm by cytase, which, according to the 

 researches of Brown and Escombe and others, is secreted by the aleuron 

 cells. After the destruction of the cell walls, simple, instead of osmotic, 

 diffusion is established, and the starch and other cell contents are exposed 

 to the uninterrupted action of the various enzymes present. 



Cytohydrolysis commences very soon after germination begins. In 

 twenty-four hours, or less, the middle lamella of the starch cells in contact 

 with the aleuron layer shows signs of dissolution, and in two or three 

 days the walls of these cells swell and separate from each other, dissolving 

 later and setting free their contents. 



The endosperm tissue is gradually converted from the outside inwards 

 into a white pasty mass, which becomes more and more liquid and limpid 

 as the starch grains dissolve, the last portions to disappear being the 

 amyliferous parenchyma around the furrow of the grain and the aleuron 

 cells. Leakage of dissolved reserves is effectively prevented by the 

 semi-permeable testa, which with the cross layer of the pericarp closely 

 overlaps the edges of the scutellum. The aleuron layer, according to 

 Haberlandt, is a glandular digestive organ secreting cytase and diastase, 

 and, according to Holland, a gluten-transforming ferment also. The 

 protein granules stored within its cells are utilised in the nutrition of the 

 protoplasts of the cells. 



The layer undergoes chemical changes more slowly than the rest of 

 the endosperm, little alteration in it being apparent during the first two or 

 three days except its separation from the adjacent starch cells. 



In about a week, when the plumule has attained a length of 3 or 4 cm., 

 the aleuron granules in many of the cells have lost their refractive char- 

 acter or have completely disappeared ; later, they vanish from all the 

 cells and the remaining brownish granular cytoplasm shrinks and dies, 

 retreating first from the tree walls next the endosperm cavity. 



The cell walls from which the cytoplasm has shrunk become crumpled, 

 and are soon attacked by the cytase, the cell cavities being then exposed 

 (Fig. 21). Ultimately, the radial and outer walls of the aleuron layer 

 are broken down and dissolved together with the hyaline nucellar layer. 

 In 10-15 days at 1^ -20" C., when the first foliage leaf of the young 

 plant is expanded, the pericarp with the adherent testa is found as a hollow 



