LEAVES STRUCTURE OR ANATOMY 



8 9 



thicker walls of the guard-cells (Fig. 114) absorb water 



from adjacent cells, these thick walls buckle or bend and 



part from each other at their middles on either side the 



opening, causing the stomate to open, when the air gases 



may be taken in and the leaf gases may pass out. When 



moisture is reduced in the leaf tissue, the guard cells part 



with some of their contents, the thick walls 



straighten, and the faces of the two opposite 



ones come together, thus closing the stomate 



and preventing any water vapor from pass- 



ing out. When a leaf is actively at work 



making new organic compounds, the stomates 



are usually open; when unfavorable condi- 



tions arise, they are usually closed. They 



also commonly close at night, when growth 



(or the utilizing of the new materials) is most 



likely to be active. It is sometimes safer to 



fumigate greenhouses and window gardens 



at night, for the noxious vapors are less 



likely to enter the leaf. Dust may clog or 



cover the stomates. Rains benefit plants 



by washing the leaves as well as by provid- 



ing moisture to the roots. Ifjs 



Lenticels. On the young woody twigs 

 of many plants (marked in osiers, cherry, 

 birch) there are small corky spots or eleva- 

 tions known as lenticels (Fig. 117). They mark the loca- 

 tion of some loose cork cells that function as stomates, 

 for green shoots, as well as leaves, take in and discharge 

 gases; that is, soft green twigs function as leaves. Under 

 some of these twig stomates, corky material may form 

 and the opening is torn and enlarged: the lenticels are 

 successors to the stomates. The stomates lie in the epi- 



fli 



FIG. 117. LEN- 



OF RED OSIER 



