658 



BOTANY OF THE LIVING PLANT 



CEREAL GRAINS. 



By far the most important vegetable foods are the Cereal Grains, 

 which are the fruits of various Grasses (Gramineae). The general con- 

 struction of all of these grains is the same, and the structure of the grain 

 of Wheat will serve to illustrate it for them all. 



The Wheat-Grain is oval, and hairy at the apical end, but smooth 

 at the base where is the scar of attachment. A lateral groove running 

 longitudinally marks the pos- 

 terior side ; the anterior side 

 is convex, and shows near its 

 base an area which is de- 

 pressed and wrinkled when 

 dry. This marks the position 

 of the germ, which is thus 

 basal and faces the anterior 

 side of the grain (Fig. 503). 

 The greater part of the grain 

 is made up of a mass of endo- 

 sperm : this together with the 

 germ is covered by the fruit- 

 coat (pericarp) and seed-coat 

 (testa), which jointly form a 

 hard brittle shell, separated in 

 milling from the inner parts 

 as Bran. 



A microscopic examination 

 shows that the germ consists 

 of thin-walled tissue densely 

 stored with oily protoplasm, 

 but with no starch. The endo- 

 sperm is also thin-walled, but 

 contains much starch closely 

 packed (Fig. 482, am) ; but a 

 superficial layer of its cells is 

 distinguished by the absence of starch, while as it contains numerous 

 aleurone grains it is recognised as the aleur one-layer (al). The Bran 

 consists of compressed and thickened cell-walls of woody texture, 

 containing a deposit of silica. The outer band (p) represents the 

 fruit-coat or pericarp : the inner (/) represents the seed-coat or 

 testa. 



The constitution of the Wheat-Grain as shown by analysis may vary 

 considerably according to sample. This is shown even in so important 

 a feature as the proportion of nitrogenous substances. " Soft Wheat ' 

 may contain only io-8o per cent, of protein, while " Hard Wheat " has 

 been found to contain 13-83 per cent. In some Russian Wheats it 

 may even rise above 17 per cent. These facts are mentioned to show 

 that the results of analysis must be taken as a general guide rather than 

 as a statement of constant fact. 



Fig. 503. 



Part of a median longitudinal section of a grain of 

 Wheat, showing embryo andscutellum (sc). us = vasc. 

 bundle of scutellum"; ce = its columnar epithelium; 

 r=ligule; c=sheathing part of cotyledon; £y=vege- 

 tative cone of stem ; &/> = hypocotyl ; / = epiblast ; 

 r = radicle; cl = root-sheath ; m = micropyle ; /^funi- 

 culus; vp = vascular bundle of funiculus; /= lateral 

 wall of groove ; c/> = pericarp. ( x 14.) (After Stras- 

 burger.) 



