16 



THE BOOK OF WHEAT 



6y the pericarp, which is also composed 'of three layers, all 

 colorless. The exterior of these three membranes, the cuticle, is 

 easily removed by rubbing. Then come two layers of cellular 

 tissue, the epicarp (from which spring the hairs above men- 

 tioned) and the endocarp. The tegmen and testa form about 2 

 per cent of the weight of the grain, and the pericarp forms 

 fully 3 per cent. Thus the bran forms at least 13 per cent of 

 the grain. 



Germination. The three conditions essential to the germin- 

 ation of wheat are moisture, warmth and oxygen. In the ab- 

 sence of any one 

 of these the 

 process will not 

 begin, or if it has 

 begun it will 

 cease. Johnson de- 

 fines the period 

 of germination as 

 lasting from the 

 time when the 



rootlet becomes 

 Cross section of grain of wheat on the left. (From _ -i i , .-i .1 n 

 micro-photograph by Tolman.) Transverse section, visioi 

 on the right, of an unripe grain enlarged about UK) cforps; n f t h P 

 times from drawing by Bessey. 1. ovary wall or peri- fe 



carp; 2, outer integument; 3, inner integument; 4, mother seed are 

 remains of nucellus ; 5, aleurone cells ; 6, starch cells. . , _ 



exhausted and the 



young plant is wholly cast upon its own resources. At 41 F., 

 the time required for the rootlet to appear in wheat is about six 

 days, which time corresponds to the more general idea of the 

 period of germination. At 51 this time is shortened about 

 one-half. The time required for the completion of germination 

 is 40 to 45 days at 41 to 55 and 10 to 12 days at 95 to 100. 

 The lowest temperature at which wheat will germinate is 41, 

 the highest 104, and that of most rapid germination, 84. 

 This is according to Johnson. Other authorities claim 

 that wheat will germinate and grow on melting ice. 

 It has also been said that it does not germinate suc- 

 cessfully at a high temperature, and consequently should not 

 be sown until cool weather in southern climates. Dissolved 

 salts seem to aid germination under ordinary field conditions. 

 In germinating, wheat absorbs from five to six times its 



