XXIV IN1"R0DUCTI0N TO BOTANY. 



155. A yoimg and latally active cell consists of the following; parts : — 1, 

 the outer wall, a peimcable, transparent membrane, formed of a chemical 

 substance called ccllHlose ; 2, a n\ucilag-inous film lining the wall, and 

 called " the jjn'mordial utricle ; " 3, the hhcIcus, or centre of cell-function or 

 life, a soft, subgelatinous body occujiying the middle of the cell, or ex- 

 centrical ; and 4, a viscid fluid, called protoplasm, filling the space be- 

 tween the nuchas and the j)iimordiul utiiclo. As the cell increases in 

 size, its contents change ; and finally, when it has attained its proper 

 dimensions, the wall formed of cellidose alone remains as a persistent 

 fabi-ic, the nucleus is absoibed or dried up, and the protoplasm passes out 

 into younger cells. 



156. The pi-incipal organized contents of cells are : — 



{a) sap, the first product of the digestion of the inorganic food of 

 plants : it contains the elements of vegetable gro-v^-th in a 

 dissolved condition. 



{b) sugar, of which there are two kinds, cane-snrjar and grape-sugar^ 

 usually exists dissolved in the sap. It is found abundantly in 

 growing parts, in fruits, and in germinating seeds. 



(c) dtxtrine, or vegetable mucilage, a gummy substance intennediate 

 between sugar and starch. 



{cV) starch or fectda, one of the most universal and conspicuous of 

 cell-contents, and often so abundant in farinaceous roots and 

 seeds as to fill the cell-cavity. It consists of minute grains, 

 called starch-grahules, which vary in size and shape, and are 

 marked with more or less conspicuous concentric lines. Starch 

 is unaffected by cold water, but forms a jelly with boiling water, 

 and turns blue when tested by iodine. 



(<?) chlorop/ii//l, the green matter of plants, is of a resinous nature, 

 and contains nitrogen. It is formed only under the action of 

 sunlight, and is usually most abundant in the layers of cells 

 immediately below the surface. 



(/) chromule, a name given to a similar colouring-matter when not 

 green. 



(g) wax, oils, camphor, and resinous matters are common in cells ; 

 also various mineral substances, either in an amorphous state 

 or as microscopic crystals, when they are called r aphides. 

 These last are peculiarly abundant in the tissues of the Cacti 

 and Rhubarb. 



\ 2. The Epidermis and its Frocesses. 



157. The Epidermis, or outer skin of plants, is formed of one or more 

 layers of vertically flattened, fimily coherent, and usiially empty cells, 

 with thin and transparent, or with thick and opaque walls. It covers all 

 parts exposed to the air, except the stigma and glands ; but is absent in 

 parts submerged under water. It serves to protect the tissues from the 

 immediate action oi the air or of drought. 



158. The epidermis is pierced by minute spaces between the cells, called 

 stovrates. They are oval or mouth-shaped, bordered by lips, fonnedof two 

 or more elastic cells so disposed as to cause the stomate to open in a moist, 

 and to close up in a dry state of the atmosphere. They communicate with 

 intercellular ca\dties, and are obviously designed to regulate evaporation 

 and respiration. Stomates are found abundantly on leaves, especially on 

 the lower surface ; in succulent leaves, though abundant, they are often in 

 an imperfectly organized condition, and seemingly inactive. 



150. ]l((irs ia(; minute, transparent expansions of cellular tissue, pro- 

 ceeding fiom the epidermis. The haiis of roots are active absorbents ; 



