74 NATURE TEACHING. 



fresh ; but if there is none or only very little 

 water to be obtained, as in the cut shoot or 

 the plant in dry ground, the roots cannot take 

 up enough to make up for what the leaves give 

 off, and first the leaves and afterwards other 

 parts of the plant droop and wither. 



3. In transpiration the green spongy tissue 

 of the leaf gives off moisture which escapes 

 into the outside air by the minute openings 

 in the surfaces of the leaf. These openings 

 are able to open and close according to con- 

 ditions, and so regulate the rate at which 

 water can be given off. When the air is dry 

 they become smaller and so hinder the escape 

 of water. We shall see, too, from our practical 

 work that light has . an important effect, and 

 that plants give off more water when exposed 

 to the light than when in the shade. When 

 cuttings of plants are being taken the shoots 

 are separated from their roots and cannot 

 obtain much wat3r. It will be clear now why, 

 under such circumstances, some of the leaves 

 should be cut off and the cuttings placed in 

 the shade. 



4. In a long drought there is insufficient 

 water to counterbalance that given off by 

 the leaves, and, although the pores may be 

 closed, there is danger of injury to the plant 



