62 



BIOLOGY FOR BEGINNERS 



wer MOSS 



the cells, which are therefore robbed of their water, since the 

 osmotic flow is toward the liquid of greater density. This fact is 

 often utilized in killing weeds and grass along the sidewalks. 



Variations in Osmosis. Osmosis hi roots is affected by the 

 temperature and amount of moisture in the soil, being less in cold, 

 dry seasons. Also the presence of organic acids in bogs, or of 

 certain mineral matters in some soils, tends to hinder or prevent 

 the process. Hence it follows that in our cold season, most plants 



shed their leaves, so that 



C-E T8 P / 5 ^ tnev nave ^ ess sur f ace fr m 



~~ which to evaporate water, 



because their supply is cut 

 down by the cold. 



In the case of both bog 

 and desert plants, many 

 schemes to retain moisture 

 have developed. Though 

 in such different surround- 

 ings, both classes of plants 

 have difficulty in absorbing 

 enough water, because of 

 the stoppage of osmosis. 



Aerial roots find even 

 greater difficulty in obtain- 

 ing sufficient water, and 

 many wonderful devices 

 have been developed in 

 FIG. 10. Compare the position of root the way of hairs to radiate 



and of stem in A and B. heat, scales to catch water, 



and enormous, thickened 



cortex to retain it when once it is absorbed. 



EXPERIMENTS WITH ROOTS 



To Prove that Roots turn toward Gravitation. If well-started 

 seedlings be inserted in a split cork which is then put into a test 

 tube of water and inverted, it will be found that the upward 



PLANT INVERTE O 



ROOT3 TURAHNfr DOWM 

 TURNING UP 



(pQS -) 



