INTRODUCTION. 



19 



Fig. 7. 



anastomose with each other, and are 

 remarkable for their irregular con- 

 tractions and expansions. Their 

 sides are very delicate, and their 

 average diameter is not more than 

 one fourteen-hundredth of an inch. 

 These vessels serve for the passage 

 of the nutritious sap or proper juice 

 of the plant; they are found in the 

 under surface of leaves, and in the 

 bark. 



Every portion of the external sur- 

 face of a plant is covered by a cuti- 

 cle or epidermis, which consists of 

 layers of cellular tissue, in which the cells are much flattened, and in close 

 contact with each other. Their 



Lactiferous Vessels. 



form 



Fig. 8. 



are oblong and 



Stomata. a Cells of Stoma, 

 c Opening of Stoma. 



Cells of Parenchyma. 



Fig. 



various, sometimes they 

 regular, whilst at 

 others they are very dissimilar, 

 and appear to interlock with each 

 other. This cuticle is provided 

 with small apertures, usually of an 

 oval form, and bounded by two or 

 more oblong or reniform cells 

 placed side by side ; and by their 

 expansion or contraction, the open- 

 ing is increased or diminished. These apertures are called stomata, and ap- 

 pear to subserve the functions of respiration and evaporation. They are 

 chiefly found in the soft green tissue of leaves and young shoots, but may 

 also be detected in the flowers. 



Glands are organs designed to elaborate peculiar secretions from the vege- 

 table fluids. They are 

 found on all parts of plants, 

 and are of various forms 

 and composition; some be- 

 ing composed merely of 

 cellular tissue, whilst 

 others contain in addition, 

 a large number of vessels 

 of different kinds. They 

 have received various 

 names, according to their 

 form, but are not fully 

 understood. Sometimes 

 they are combined with 

 hairs, which are expan- 

 sions of the cellular tissue 

 of the cuticle, consisting 

 either of single elongated 

 cells ; of several cells 

 placed end to end; or of 

 cells branching from each 

 other. These hairs may arise from a gland, or bear one at their apex. 



Hairs and Glands of various kinds. 



