74 OFFICE AND FORMATION OF 8TOMATA. 



ture is nearly the same on the two sides, and the stomata are 

 equal in number. Again in Plants, the circumstances of whose 

 growth are such, that the atmosphere commonly comes in contact 

 with the upper side only of the leaf, as in the case of the Water 

 Lily, the leaves of which float on the surface of the water, the 

 stomata are disposed on that side alone. 



93. As there is no cuticle to protect the tissues of plants grow- 

 ing altogether beneath the surface, so there is no occasion for 

 stomata to admit the passage of air to these ; and accordingly in 

 the whole tribe of Sea- weeds we find no vestige of them. Neither 

 can they be distinctly traced in the Mushroom tribe, nor in 

 Lichens ; but in the Liverworts they present themselves, in the 

 most remarkably complex form which we anywhere witness ; in 

 the Mosses they have only been detected on the stalk which bears 

 the fructification ; whilst in most Ferns, as well as in Flowering 

 Plants, they abound. 



94. Of the very minute size of these curious organs, some 

 idea may be formed from the fact, that in some leaves it is esti- 

 mated that 70,000 occur in a square inch of cuticle. The largest 

 known are about the 1-500 of an inch in length ; whilst the 

 smallest are not 1-3000. Their function is evidently to allow of 

 that limited evaporation of water from the soft tissues of the 

 plant, which will hereafter be shown to be one of the most im- 

 portant of the processes, by which the crude fluid absorbed by 

 the roots is converted into the nutritious sap or proper juice. 

 The influence of light upon the stomata causes them to open, 

 whilst they contract and even close in darkness. 



95. It has also been shown, that light has a most important 

 influence on their first production. In the young plant of the 

 Marchantia (. 33), when first separated as a kind of bud from 

 its parent, no stomata or roots exist. It lias been ascertained by 

 repeated experiments, that stomata and roots may be caused to 

 develop themselves in either of the two sides ; the stomata 

 being always formed on the upper surface, under the influence 

 of light, and the root-fibres proceeding from the lower towards 

 darkness. But if the surfaces be reversed after the respective 

 organs have been developed to a certain point, so that the stomatu 



