Plant Study 9 



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plants the epidermis is replaced by bark. Beneath the bark 

 is a layer of cells called the cambium-layer, forming the growing 

 tissue of the plant cylinder. 



Minute openings, known as stomata (singular, stoma), are 

 found in the epidermis of the leaves. These openings are 

 extremely small and the number on a leaf is very large ; it has 

 been found that more than one hundred thousand are present on 

 the under surface of an apple leaf. The water that passes from 

 the plant as vapor and the oxygen set free in the elaboration 

 of food in the leaves escape from these openings, and carbon 

 dioxide from the air passes into the plants through them. 



Function of roots. The roots have two very important 

 functions. They anchor the plants in the ground and serve 

 to supply them with water in which is dissolved the food that 

 is taken up from the soil. The root system is made up of the 

 main roots and branching parts that penetrate the soil in all 

 directions. On these small branching parts are found tiny 

 root-hairs that penetrate between the soil particles and absorb 

 water containing plant-food, which is carried up into the plant 

 as sap. As many as twenty to twenty-five thousand root- 

 hairs may be present on a square inch of root surface. Each 

 root-hair consists of a single elongated cell. As the end of 

 the root advances through the soil, new root-hairs are formed 

 beyond the older ones and those farther back die. 



Water from the soil passes through the cell-walls of the root- 

 hairs by what is known as osmotic pressure. When two liquids 

 of different densities are separated by a semi-permeable mem- 

 brane, there is a movement of the less dense solution toward 

 the more dense. This is known as osmosis. That liquids 

 move as just described can be proved by tying a piece of pig's 

 bladder that has been soaked in water over the end of a thistle- 

 tube, filling the tube with a sugar sirup until it stands in the 

 neck of the tube, and placing the tube, bell-end down, in the 

 water. A large-mouthed bottle fitted with a cork through which 

 the tube can extend is a convenient receptacle to hold the water. 



