38 BOTANY. [chap. i. 



leaves, -whicli in the spring contain a very small trace of 

 ash or mineral matter, whereas at maturity most of the 

 cells contain quantities of crystals, 



(4) Cell-sap. This substance saturates the cell-wall 

 and protoplasm, and also fills the central sap-cavity in 

 the fully-developed cell. It is a watery solution of 

 various substances, amongst which certain salts derived 

 in solution from the soil are never absent; in certain 

 cells of some plants, as sugar-cane, beet, maple, etc, 

 large quantities of cane-sugar are present; in certain 

 cells of the grape, and many other fruits, grape-sugar 

 occurs; in addition, tannin and many vegetable acids 

 are present, as also many of the blue and red colouring- 

 matters of flowers and fruits. 



The epidermis or protective covering formed on the 

 surface of those parts of plants growing surrounded 

 by air, usually consists of a single layer of cells that 

 become differentiated in the following manner. The 

 cells touch each other at every part, intercellular spaces 

 being entirely absent; the outermost, or free surfaces 

 of the epidermal cells become much thickened and con- 

 verted into a substance impervious to water ; the proto- 

 plasm and chlorophyll usually disappear at an early stage, 

 their place being taken by air or water, the epidermis 

 serving as a reservoir for the latter in many plants. At 

 a very early stage of the development of the epidermis 

 the stomata, or openings through its substance, are 

 formed. Their mode of formation varies, to a certain 

 extent depending on the particular species examined. 

 If the tip of a very young leaf that is just showing at 

 the crown of a growing hyacinth bulb is removed, and 

 a minute portion of its epidermis removed with a pair of 



