io6 



EPIDERMIS. 



[CHAP. 



equilibrium, until it reaches the leaves and other tissues 

 exposed to evaporation. The elaborated sap, as it has 

 been called (that is, the fluid cell-contents, containing avail- 

 able nutrient matter derived from the chlorophyll-contain- 

 ing cells), is transferred through the latticed tubes of the 

 phloem of the vascular bundles to wherever it may be 

 required for present use or storage. If you remove 

 the ring of phloem from the stem of a tree^ or bind it 

 very tightly round with a strong hoop, no wood will be 

 formed below the ring or hoop. On the other hand, a con- 

 siderable thickening will take place immediately above it. 

 From the absence of a system of vessels analogous to 

 that of animals, and of a pumping-engine like the heart. 

 the course taken by fluids in plants is comparatively very 

 vague and ill-defined at best. I have here merely indi- 

 cated its general course in the stems of Dicotyledons. 



14. I have spoken of leaves as capable of absorbing 

 gases, especially carbonic-acid gas, and probably also 

 vapour, from the atmosphere. 



If a leaf be examined carefully, it will be found covered 

 with a thin skin or epidermis, which very often (in fleshy 

 leaves) may be torn off in filmy shreds. And a similar 

 epidermis covers nearly all the green 

 and coloured organs which are exposed 

 to the air. If a piece of this epidermis, 

 torn from a leaf with the thumb and a 

 sharp penknife, be placed in a drop of 

 water upon a glass slide, its structure 

 may be easily made out under the 

 microscope. Suppose a shred torn 

 from the leaf of a Hyacinth. It will be 

 found to consist of an excessively thin 

 layer of flattened cells, closely fitting 

 at their angles. Scattered at intervals 

 over the epidermis are pairs of very 

 small cells side by side, with their 

 ends in contact, as shown in the cut. 

 Each pair of cells forms a stomate. 

 If the epiderm of a growing leaf of 

 Hyacinth be examined from apex to 

 base, stomata in various stages of development may be 

 found, from the first appearance of small squarish green 



FIG. 72. Epidermis 

 of Hyacinth, show- 

 ing one open sto- 

 mate. 



