516 



WEED FLORA OF IOWA 



SCh. 



Fig. 394. Broom-corn. Layers of the pericarp in surface view, 

 of letters same as in Fig. 391. X 160. 

 (After Winton, Conn. Agr: Exp. Sta.) 



Significance 



face view. 2. The Hypodcnn consists of from one to three layers 

 of cells, with walls somewhat thinner than those of the epidermis. 

 3. Starchy Mesocarp. Several layers of thin-walled parenchyma 

 cells, filled usually with small round or rounded polygonal starch 

 granules seldom over 0.006 mm. in diameter, make up this coat. 

 In all the varieties here described the starch appears during the 

 early stages of growth and persists until the fruit nearly or quite 

 reaches full maturity. As the caryopsis, even when nearly mature, 

 is intensely green owing to chlorophyll grains in the outermost 

 layers of the mesocarp, it may be inferred that this starch is a 

 direct product of photosynthesis in the pericarp. So far as I have 

 observed, the presence or absence of a starchy mesocarp in the 

 grain at the time of harvest is not a definite varietal peculiarity, 

 but is dependent on the ripeness of the fruit or other conditions. 

 Some kernels of the same variety may possess it, while others show 

 only empty, obliterated cells. "Whether or not the starch is present 

 in a given seed may often be determined by careful scraping and 

 observation with the naked eye. 4. Cross-Cells. These cells are 

 usually long and narrow, being distinguished from the tube-cells 

 only by their transverse arrangement. Near the extremities of 

 the seed they are, however, shorter and of more irregular shape. 

 5. Tube-Cells (sch). The cells of this layer lie at right angles to the 

 cross-cells. They are about 0.005 mm. wide and often reach a 

 length of 0.20 mm. a. Nucellar or Hyaline Layer. This layer is 

 frequently 0.05 mm. thick. The outer radial walls are thin, but 

 the inner wall is greatly swollen. In surface view the large cells 



