20 GENERAL BOTANY 



any loss in weight at the end of the day, from 

 which we learn that it is almost entirely through the 

 leaves that water passes out into the air. 



We see, therefore, that the water in the plant is not 

 stationary, but is moving up from the soil through 

 the stem and branches to the leaves, and from the 

 leaves passes out into the air as water-vapour. The 

 actual amount passed up differs, of course, very much, 

 depending on the size of the plant and the number 

 of its leaves, and their nature. Plants which, as 

 a rule, inhabit dry places lose comparatively little 

 water ; those which naturally live in damp soil re- 

 quire a great deal. 



Empty the pot that you have weighed, and after 

 separating the earth carefully from the rootlets, put the 

 plant and soil back again and water it. You will 

 probably find the next day that the leaves are droop- 

 ing, however much water you put, and the plant will 

 very likely die. This is because in ordinary soil water 

 does not occur in visible amount, but as very thin 

 films round each particle of soil. It is only the root- 

 hairs, because they attach themselves closely to the 

 smallest particles of soil, that can get this water, and 

 therefore if they are broken the plant is unable to 

 get water (unless the soil is very wet) and therefore^ 

 suffers. 



5. We have already seen that comparatively little 

 w^ater is evaporated from the stem and branches, 

 practically all that is lost passing out from the 

 leaves. But even on these it is not by the whole 

 surface, for they are covered by a very thin but 

 water-proof membrane called the epidermis in which 



