MICROSCOPIC STRUCTURE OF WEED SEEDS 



523 



Fig. 401. Darnel. Transverse section of fruit. F, pericarp consisting of 

 epidermis ep, mesocarp m, cross-cells q, and tube-cells sch ; S, testa con- 

 sisting of outer layer a. and inner layer i ; N, perisperm ; f, fungus layer ; 

 E, endosperm consisting of aleurone layer al, and starch-parenchyma st. 

 X 160. 



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



transverse sections this coat often separates from the pericarp on 

 the one hand and the perisperm on the other. Examined in water, 

 only one cell layer (the inner) is evident; but successive treat- 

 ments with 5 per cent potash, dilute acetic acid and chlorzinc 

 iodine, bring out two layers. 1. The Outer Layer (a) is made up 

 of thin-walled cells with cuticularized outer walls. Treated as 

 above described, the cuticle is colored yellow-brown, the radial and 

 inner walls, blue. 2. The Inner Layer (1) is not only thicker than 

 the outer, but the cells are thicker-walled and, in addition, swell 

 greatly with potash. These swollen walls are stained deep blue by 

 chlorzinc iodine, thus differentiating them from the yellow-brown 

 cuticle on the inner wall. 



Perisperm (N). — Characteristic of this seed is the nueellar-coat, 

 consisting usually of two cell layers. In cross section these cells 

 are rectangular with swollen walls; in surface view, as may be 

 seen after soaking for a long time in dilute potash, they are ir- 

 regularly polygonal or more or less elongated. 



Fungus Layer (F) . — In most specimens a layer of fungus-threads 

 20 thick is present between the perisperm and the aleurone layer. 

 So commonly is this fungus present in darnel grown in Europe, 

 that it is of no little value in identifying the grain ; but it remains 

 to be determined whether in California, where the plant is a pest 



