30 



EXPEBIMMNTS WITH PLANTS 



of iruys. Perhaps we cannot say in all cases which 

 solution is the best: cei'tainlj', in absorbing water the 

 thinnest and most permeable cover is the best, but 

 whether such a cover would in all cases furnish suf- 

 ficient protection to the seed before germination we 

 cannot tell without a special study of that question. 



We know that, besides 

 water, air is needed. How 

 does the germ get air ? Does 

 air pass readily through the 

 seed- cover? We may find 

 out by sealing the cover 

 (air-tight) to one end of a 

 glass tube, filling the tube 

 with water and inverting it, 

 as shown in Fig. 28, in a 

 glass of water, taking care 

 not to admit any air into 

 the tube; the water should 

 be boiled just before using, 

 in order to expel the air. 

 To seal the tube to the 

 cover, smear the end of the 

 tube liberally with hot sealing-wax; remove the super- 

 fluous wax from the interior of the tube, press the end 

 of the tube firmly against the cover (which must be 

 free from cracks or openings), and run a hot v\-ire 

 around it until a tight joint is secured; allow the wax 



28. :\Iethod of testing the permeability 

 of the seed-cover to air. 



