26 THE WHEAT PLANT 



generally more speedy in these grains on account of the thinner pericarp, 

 which permits of rapid absorption of water. 



When whole spikelets of the " Spelt " wheats are sown, the coleorhiza 

 and rootlets of the embryo pierce the base of the grain and the enclosing 

 flowering glume, but are unable to break through the thicker empty 

 glumes, the entrance of the rootlet into the ground being made past the 

 edges of the latter at the base of the spikelet. 



In these wheats, in which strong glumes closely invest the grain, 

 no longitudinal slit exposing the plumule is torn in the pericarp by the 

 expanding coleorhiza and rootlet. 



In some cases the flattened wedge-like coleoptile breaks through the 

 pericarp near the upper edge of the scutellum and appears at the tip of 

 the spikelet, after growing along the space between the outer surface of 

 the caryopsis and the inner surface of the flowering glume : most fre- 

 quently, however, the plumule escapes from the apex of the grain after 

 pushing its way longitudinally within the substance of the pericarp in 

 which it travels through the thin parenchyma outside the " cross layer." 



Placed in the soil or on damp filter paper, wheat caryopses soon 

 begin to absorb water, which penetrates most rapidly through the thin 

 portion of the pericarp covering the embryo. Laid furrow side down on 

 a wet surface, they imbibe more slowly than when the dorsal half is wetted. 



In order to test the amount absorbed, 100 grains were completely 

 immersed in water. At certain intervals they were withdrawn, and after 

 the superfluous water had been carefully removed from their surfaces 

 by means of filter paper, were weighed again. They were then immersed 

 for a further period. The results obtained are shown in the table on 

 the following page. 



Germination commenced after about 40 hours' immersion, up to which 

 time the amount of water absorbed by the grains steadily increased, the 

 flinty grains imbibing more slowly than the starchy ones. 



From another series of investigations I found that grains do not 

 sprout until they have absorbed at least 30 per cent of their weight of water, 

 and, with the water content maintained at this figure, germination is 

 feeble and protracted. 



The coleorhiza emerges in about 24 hours after the grains have taken 

 up 35 per cent of water, which they usually do after 12 to 14 hours' soaking 

 at 17-18 C. The most vigorous germination, however, at this tem- 

 perature takes place when the amount imbibed is 36-40 per cent of 

 the air-dry weight of the grains. 



In addition to the increased weight due to the absorption of water, 

 there is always a change in the volume of the grain, the swollen caryopsis 

 ready to germinate being from 35 to 40 per cent larger than when 

 air-dry. 



