466 BOTANY 



In the " vascular " plants, i.e. Pteridophytes and Spermatophytes, 

 the vascular bundles are of great importance in the transportation 

 of food solutions. The woody tissue, composed principally of dead 

 tracheary elements, is the main channel for the passage of water 

 absorbed by the roots. The mechanism of those movements is still 

 imperfectly understood, but the capillary action of the empty tubular 

 tracheae is doubtless of great importance in the process. 



Movements of Gases 



Certain gaseous substances, notably free oxygen and carbon-dioxide, 

 are of the first importance in the life of normal plants. It is true 

 that certain anaerobic Bacteria are independent of free oxygen, and 

 plants without chlorophyll do not use CO 2 , but in all green plants 

 both of these gases are necessary. 



The entrance of gases into the cell is always by diosmosis, and 

 can only take place through the cell-wall, when it is saturated with 

 water. Through a dry membrane no diffusion of the gases occurs. 

 In submersed aquatic plants the gases, like other substances held in 

 solution in the water, may enter through the outer membrane of the 

 superficial cells. In aerial organs, however, like the leaves of land 

 plants, the cuticularized membranes of the epidermal cells prevent 

 the entrance of gases, which must first enter the intercellular spaces 

 through the stomata. 



Osmotic Pressure in the Cell 



The substances dissolved in the cell-sap attract water osmotically 

 with a force proportioned to the differences in concentration of the 

 cell-sap and the solution outside the cell. If the amount of osmoti- 

 cally active substances in the cell-sap remains unchanged, either by 

 not escaping from the cell, or by being constantly renewed, there will 

 be a continuous absorption of water, and the volume of the cell will 

 tend to increase proportionally. The internal pressure will force the 

 protoplast into close contact with the cell-wall, and the latter, being 

 more resistant than the protoplast, will become stretched and tense. 

 This tense condition of the cell is known as " Turgescence " ; the 

 stretching force which produces it, " Turgor." This pressure within 

 the cell may amount to five to ten atmospheres, and under certain 

 conditions, as when pollen-spores are placed in water, the pressure is 

 sufficient to burst the cell-membrane. The turgescence of the normal 

 cells gives the plant-tissues their elasticity and firmness, and when 

 the turgescence is diminished by loss of water from the cells, as hap- 

 pens, for instance, when plants are exposed to the hot sun, the droop- 

 ing of the leaves, or "wilting," is the result of this diminished 



