18 A HALF-HOUR WITH THE 



PL 1). The object of this tissue is, evidently, to 

 allow of the existence of a large quantity of air 

 in the spaces between the cells ; by which means 

 the stem of the plant is lightened, and it is better 

 adapted for growth in water. 



If the leaf of any plant is examined, it will be 

 found that on the external surface there is a thin 

 layer, called, after the thin external membrane in 

 animals, the " epidermis." This layer is composed of 

 very minute cells smaller than those in other 

 parts of the plant, and when placed under the 

 Microscope, presents a variety of forms (Figs. 37, 

 38, 39, 40, 41, PL 2) of cellular tissue. There is, 

 however, a peculiar structure in this layer, which is 

 found on the outside of all parts of plants, which 

 demands attention. In the midst of the tissue, at 

 very varying distances, are placed little openings, 

 having a semilunar cell on each side. These open- 

 ings are called " stomates," and can be well seen in 

 the leaf of the hyacinth, where the cells of the epi- 

 dermis are transparent ; but the little cells which 

 form the stomate are filled with green colouring- 

 matter (Fig. 37, PL 2). The stomates vary very 

 much in size and in numbers. They are found in 

 larger numbers on the lower than 011 the upper 

 side of leaves. In the common water-cress they 

 are very small, and the cells of the epidermis are 

 sinuous (Fig. 38, PL 2). The stomates are found on 

 all plants having an epidermis. In Figs. 39 and 41 

 they are represented from the wheat and the aloe. 

 In the latter plant the cells of the cuticle are very 

 much thickened. They can also be seen on the 

 cuticle of the fruit (Fig. 231, PL 8) and the petals. 

 This part of the plant forms a beautiful object 

 under the Microscope. The petal of the common 

 scarlet geranium (^Pelargonium) affords a beautiful 

 instance of the way in which the cells of plants 



