140 THE PRINCIPLES OF FLORICULTURE 



these change their position, due to the movement 

 of the protoplasm within them, and openings 

 appear in the epidermis. These admit air, 

 which is so necessary for healthy plant growth, 

 to the interior of the leaf. Under other condi- 

 tions, this protoplasm again changes its position 

 and the stomata close. These stomata are in- 

 timately connected with air spaces between the 

 spongy cells on the lower side of the leaf. 

 (k) Palisade cells. In the palisade cells, active food 

 manufacture takes place. The protoplasm of 

 these .cells contains many chloroplasts. To 

 this protoplasm with its green plastids, crude 

 food materials are brought from the root-hairs 

 through the various cells. The passage of the 

 liquids is due to osmosis or to root-pressure 

 due to cell growth in roots, and to other 

 factors. These food materials are combined 

 with the air which has entered the leaf through 

 the stomata and which is more or less charged 

 with carbon dioxide. By the action of sunlight, 

 these plastids change the crude food substances 

 into sugars, starches and other compounds 

 which stimulate protoplasm into growth. They 

 are transferred from the leaf to the tips of the 

 branches, to the roots and to the cambium layer 

 just beneath the bark, where active growth is in 

 process. If it be at the approach of a dormant 

 period of plant growth, the manufactured plant 

 food changes its chemical character and goes 

 into storage cells, there to remain until the 

 next period of active growth. 

 The number of rows of palisade cells varies, 



