268 



THE MAINTENANCE OF THE INDIVIDUAL 



open when the guard cells become more than normally turgid, but if 

 the turgor of all of the living cells of the leaf is reduced by water loss, 

 then the stomata seem to close automatically. 



Light increases the amount of sugar formed in the guard cells 

 because of the chloroplasts present, which results in a concentration 

 of sugar, thereby causing a change in turgor. When the leaf is not 

 illuminated by direct sunlight, or at night, the amount of sugar con- 

 centration in the guard cells becomes less, and consequently the 

 stomata close. They usually are closed at night but remain open 

 from shortly after sunrise until late in the afternoon. Toward the 

 middle of the afternoon they begin to close, thus decreasing the 

 amount of water lost in the latter part of the day. Plants wilt on 

 hot, dry days because they cannot obtain water rapidly enough from 

 the soil to make up for the loss through the leaves. Many adapta- 

 tions are found in the leaves which help prevent this water loss, such 

 as waterproofing of the outer cells, hairs on the leaf surface, the 

 absence of leaves, as in the cactus where the minute leaves are early 

 replaced by spines, or the actual turning of the leaves in order to 

 place a small surface to the sun, as in the compass plant, thus causing 

 the rate of evaporation to decrease. 



W riylu J'icrcc 



Capillary tubes of various sizes. Is there any relation between the size of 

 the bore of the tube and the water level in the tube ? Explain. 



