250 THE PHYSIOLOGY OF PLANTS 



watering. Even if the seeds are not sown too deeply, they 

 may not germinate if tlie soil becomes caked by repeated 

 wetting and drying, &c. Such caking of the soil prevents 

 the access of air to the seed at the time when the greatest 

 amount of chemical change is taking place within them, 

 and the air actually contained in their tissues is not sufficient 

 for any considerable length of time. 



How essential the air contained in the tissues of the seed 

 is for germination, may be gathered from experiments which 

 were made with the seeds of turnips. These were placed 

 under the receiver of an air-pump, and the air they contained 

 was pumped out and replaced by water. The seeds took up 

 71.13 per cent, of water, but only 30 per cent, of them 

 germinated, while control-experiments showed that 90 per 

 cent, of the normal seeds germinated. These latter also 

 developed much more rapidly. In adult plants, too, death 

 may be brought about by extracting the air from the inter- 

 cellular spaces where it is contained, and replacing it by 

 water. The plants then have a transparent appearance. 



A disturbing effect is often caused in seeds which have 

 been swelled, by an interruption of the process of germina- 

 tion by drought. The injuries thus caused vary according to 

 the nature of the plant and the stage of germination at which 

 the disturbance occurs. Speaking generally, the seeds of 

 Monocotyledons seem to suffer less than those of Dicoty- 

 ledons. The naked grains, like those of the wheat or rye, 

 are especially resistant. Those covered with glumes (bracts), 

 like those of the barley and oat, are more sensitive, and the 

 Indian-corn is especially delicate. Still, all these grains 

 suffer less than the linseed, rape, clover, peas, and beans. 

 Experiments made with a view to studying this phenomenon 

 showed that the cereals are able to withstand even repeated 

 interruptions of their germination by constantly forming new 

 adventitious roots at the base of the old ones, which died 

 off. This faculty is not possessed by most Dicotyledons ; the 

 dried-up roots rot away, and their decay affects the adjoining 

 tissue and spreads upwards. Even when decay does not set 

 in, and the seedling gradually recovers, it takes place much 

 more slowly. 



