FRUITS AND SEEDS 249 



the embryo is provided with a shield-shaped mass of cells, 

 the scutellum, which is applied to the starch-containiiig 

 tissue. The outer cells of the scutellum have finger-like 

 processes, and constitute the absorptive cells, taking up the 

 food material and passing it on to the embryo. 



During this period of germination we must take care that 

 there should never be a dearth of oxygen or a superfluity 

 of carbonic acid. No gas is more injurious to germination 

 than carbonic acid. If there is an admixture of only a 

 few hundredth parts of carbonic acid to one part of oxygen, 

 the germinating process will cease. 



Most of the mistakes in germinating occur from the fact 

 that there is a dearth of oxygen and an overplus of carbonic 

 acid. The direct cause of this is either too thick a covering 

 of soil over the seeds, or a closing of the soil to the free 

 access of oxygen by too continuous watering. There can be 

 no fixed rule about the thickness of the covering layer; it 

 depends partly on the size of the seed, and partly on the 

 porosii^ of the soil. 



The main consideration which will enable any one to 

 determine the depth at which his seeds should be sown 

 must always be this, that the soil is the medium which, in 

 the first place, is to keep the seeds sufficiently moist for 

 germination ; secondly, and only in those cases in which 

 the seedling will continue its development on the spot where 

 it has germinated, the soil must be looked upon as essential 

 for the fixing of the plant. In horticultural practice, where 

 the young plants are pricked out, the covering of the seeds 

 with soil is not essential, if we only keep the seed-pans 

 covered with a pane of glass, or sow the seeds in an open 

 bed in a forcing-pit. But it is different in the case of seeds 

 sown in the open, where the dry winds or hot spring weather 

 may easily cause a temporary or a fatal stoppage of growth. 

 These evils must be overcome by covering the seed with 

 sufficient soil to prevent their becoming dry, but without 

 cutting off the supply of air. 



If one is forced to use sunny beds for seedlings, it is better 

 to retain the necessary moisture in the soil by covering . it 

 with loose straw, pine branches, &c., than by continual 



