138 COMPENDIUM OF GENERAL BOTANY. 



conversely, reducing the turgescence of the guard-cells tends to close 

 the pores. Besides this general statement, a few special considera- 

 tions are necessary. It has been observed that the stomata of some 

 water-plants are open at all times whether the guard-cells are tur- 



FIG. 79. Surface-view of a breathing-pore of Triticum vulgaro. Type III. 



(Open.) 

 (After Schwendener.) 



gescent or not. It must also be remembered, as has already been 

 stated, that the subsidiary cells in some cases assist in closing the 

 breathing-pores. The opening of pores is also influenced by the 

 pressure of the epidermal cells. 



Why does the turgor of the guard- cells increase ? First of all 

 sunlight is the outer agency which produces these changes. It is 

 evident also that the chlorophyll of the guard-cells enters as a fac- 

 tor in turgor. The presence of chlorophyll is characteristic of the 

 guard-cells in contradistinction to the other epidermal cells. The 

 delicate structure or other peculiarities of the guard-cells are of 

 importance in facilitating diosmosis with neighboring epidermal 

 cells (gyrnnosperms). 



The question whether warmth has an effect similar to that which 

 light produces could not be satisfactorily answered by SCHWENDENER, 

 although he does not doubt that suddenly reducing the temperature 

 to zero reduces the turgescence of the guard-cells, while raising the 

 temperature increases it. 



The mechanism of the coniferous type is still under investiga- 

 tion. 



(b) Lenticels. 



A knowledge of these structures presupposes a knowledge of 

 cork-tissue. Lenticels are lense-shaped cork-like tissue-formations 

 of the bark which have the peculiarity of always being traversed 



