32 A HALF-HOUR WITH THE 



called " vascular tissue ;" but where the cells retain 

 their primitive form, it is called " cellular tissue." A 

 very interesting form of the latter is the " stellate" 

 tissue found in most water plants, and especially 

 regularly developed in the common rush, a represen- 

 tation of which is given in figure 3 plate 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 of cellular 

 tissue. The form of epidermal cells from various 

 plants is seen in figure 42 and the following 

 figures in plate 2. There is found in this layer a 

 peculiar organ which exists on the outside of all 

 parts of plants, and 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 openings are called " sto- 

 mates," and can be well seen in the leaf of the 

 hyacinth, which is shown in figure 42, where the 

 cells of the epidermis are transparent ; but the 

 little cells which form the stomate are filled with 

 green colouring-matter. The stomates vary very 

 much in size and in numbers. They are found in 

 larger numbers on the lower than on the upper 

 side of leaves. In the common water-cress they 

 are very small, as seen in figure 43, plate 2, and 

 the cells of the epidermis are sinuous. The sto- 

 mates are found on all plants having an epidermis. 



