TRITICUM 97 



1. Outer epidermis- — one row of cells. 



2. Parenchyma layer — many rows of colorless cells. 



3. Chlorophyll layer — one row of cubical cells, sometimes 



two, and several in the groove region. 



4. Inner epidermis — one row of small cells. 



5. Outer integument — two layers. 



6. Inner integument — two layers. 



7. Nucellus — several layers of thin- walled parenchyma 



cells, all bounded by a distinct nucellar epidermis. 



The first four regions listed above constitute the ovary 

 wall (pericarp). After fertilization, marked changes take 

 place in these coats. The nucellar tissue, except its epider- 

 mis, is absorbed by the enlarging embryo. The outer integu- 

 ment (5) and the inner epidermis (4) soon disappear. At 

 first, starch is deposited in the entire ovary wall. At the 

 time of resorption of the ovary wall, deposition of starch 

 within it ceases, and its appearance begins in the endosperm. 

 Resorption of the ovary wall begins in the layer just outside 

 the chlorophyll-bearing layer and extends slowly out to the 

 epidermis. Two to four layers next to the epidermis persist 

 in the mature grain. The chlorophyll cells become longer, 

 lose their chlorophyll, and thicken their walls. In the proc- 

 ess of maturation, the ovary wall or pericarp becomes 

 firmly attached to the outer layer of the inner integument 

 of the ovule. This behavior seems to be well demonstrated 

 in all grasses investigated. The firm attachment of the peri- 

 carp to the ovule distinguishes the grain or caryopsis from 

 the achene. 



Ripening Stages. — It is customary to speak of four stages 

 in the ripening of the grain: (i) milk-ripe or green-ripe stage; 

 (2) yellow-ripe, gold-ripe, or "dough" stage; (3) full-ripe 

 stage, and (4) dead-ripe stage. 

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