WATSIDE WEEDS. 183 



stance of the leaf. These little cells are the door- 

 keepers, foT^ being hygrometric, that is, affected by 

 the presence or absence of moisture, in dry weather 

 they contract, and by shutting up the little opening 

 between them prevent the plant losing more 

 moisture by evaporation than it can afford; in 

 moist weather the reverse takes place. It would be 

 out of place here to enter into a consideration of 

 plant respiration and digestion, which go on in 

 the leaves mainly by the communication effected 

 between the plant tissues and the atmosphere 

 through these sensitive little pores ; suffice it for us 

 that we see how this cuticle of the plant, the epi- 

 dermis it is called by some, contributes essentially 

 to a function on which not only the health and 

 growth of the vegetable world depend, but also, 

 by the purification of the air, effected by vege- 

 table respiration, the health of the animal creation 

 likewise. The stomates have been alluded to with 

 reference only to the leaves ; they occur, however, 

 but not so abundantly, on most other parts of vege- 

 tables, varying greatly in number upon different 

 plants. On the roots underground, where mani- 

 festly they would be of little or no use, the stomates 

 do not occur. 



Beautiful, however, as may be the plant cover- 

 ing, whether in its own bright polished surfaces, or 

 in the transparency which permits the passage of 

 the brilliant colours which tinge the ceUs beneath 



