RESPIRATORY TISSUES. 



13 



t 



Fiff 49.— Leaves of Uva Ursi, under surface 

 showing sL stomata, chink and guard cells. 



(Vogl). 



of the epidermis. In the higher plants 

 they are composed of two guard cells, 



as practical guides in the determination 

 in the leaves of the official plauts(a). 



The number of stomata varies widely, 

 from to 40 to 300 on an average to 

 the square mm. Some leaves have as 

 many as 700. It has already been said 

 that the number on the upper and lower 

 sides may be unequal, the majority of 

 leaves having more stomata on the under 

 side. 



The function of the stomata is the in- 

 terchauge of gases and the evaporation 

 of water, the activity of these functions 

 depending upon the opening and closing 

 action of the guard cells, which have a 

 special action. When the guard cells are 

 filled with water, they become turgid, 

 thereby making the opening larger; active, 

 evaporation can then take place until the 

 turgor of the cells is gradually dimiu- 

 ished, when the opening becomes smaller 



Fig. 50.-Cross section of leaf of Mentha piperita showina: a. stoma: ^. opening with front and 

 hind chambers, o. opv^ning. t. breathing space, A. gs. guard cells with protoplasm. Ep, epidermis, c. 

 cutinized and thickened outer wall. (Tscliircli). 



generally oval in shape from a surface 

 view, between w^iiich a space is left for 

 the passage of gases into a breathing 

 cavity beneatli. (Fig. 49.) 



On cross section they are more com- 

 plicated; in the figure (Fig. 50) ss. are 

 the two guard cells, between which lies 

 the opening E. and underneath the 

 breathing space A. On looking at the 

 cross section the external cutinized layer 

 of the epidermis may be seen. This 

 varies in thickness in every plant. Be- 

 neath this in the chink of the opening lie 

 two chambers, the front and back, which 

 vary greatly in the different stomata. 



Stomata sometimes lie superficially, at 

 other times, though less commonly, they 

 lie Immersed in the tissues of the leaf. 

 These points of Tariation in shape and 

 size, and degree of cuticularization, serve 



and the stomata are thus automatically 



closed, (b) 

 In the determination of powdered 



drugs the stomata are of importance- 

 Water Pores are not properly respira- 

 tory tissues. They are similar in shape 

 to the stomata, but differ in the immo- 

 bility of the guard cells and in the fact 

 that they generally lie at the end of a 

 fibro- vascular bundle of a leaf, and 

 hence are found in greater number along 

 the edges of the leaves, 



Lenticels are respiratory tissues oc- 

 curring in the stems where the stomata 

 of the epidermis have been lost by the 

 process of secondary growth already de- 

 scribed. They occur on stems, leaves 



(a) Tschirch, Angewandte Pflanzen Anat- 

 omie, p. 434, _ 



(b) Schwendener, 

 Akadamie, 1881. 



Berichte d. Berliner 



