10 SCIENCE AND FRUIT GROWING 



Air is carefully excluded from the junction to prevent it from 

 drying up, and ere long a perfect union between the parts takes 

 place. The fruit that is produced on branches formed above 

 the junction will be that of the scion, but if suckers or branches 

 appear below the graft, the fruit on them will be that of the stock, 

 which is generally worthless. 



Outline of the Physiological Functions of a Plant 



The main functions of a plant may be grouped under five 

 headings 



(1) Respiration, or breathing. 



(2) Absorption of water. 



(3) Excretion of water, usually by transpiration. 



(4) Elaboration of plant-food under the influence of light 



(photosynthesis). 



(5) Translocation of plant-food, 



(1) Respiration, or breathing. This is a physiological process 

 which takes place in every part of a plant root, stem, leaves 

 and flowers. It is independent of light and of the presence of 

 chlorophyll. Oxygen is taken in from the air by way of the 

 stomata, lenticels or permeable cell-walls, when, much as in the 

 case of animals, waste products are oxidised, resulting in the 

 formation of carbon dioxide, which is expelled through the same 

 channels. Respiration is carried on at all times and in all 

 seasons, and does not cease, either at night or in the winter, 

 though at such times its activity is diminished. 



(2) Absorption of water. Water, with various food-salts 

 (minerals and nitrates) in dilute solution, enters the plant from 

 the soil by way of the root-hairs. It is passed on through the 

 delicate root tissues until it reaches the wood of the vascular 

 bundles, and then travels upwards through the plant by way of 

 the vessels of the wood, until it reaches the leaves. The water 

 acts, therefore, mainly as a carrier of such substances as may be 

 present in the soil ; at the same time, however, the roots have a 

 certain power, by means of osmotic action, of absorbing some of 

 these substances to a greater extent than others. 



(3) Excretion of water, or transpiration. The bulk of water 

 needed to convey the requisite amount of food for an actively 

 growing plant is very great, and the excess is got rid of by trans- 

 piration, whereby water vapour is passed out from the cells into 

 the intercellular spaces, and thence into the outer air by way of 

 the stomata. In very young or delicate parts of the plant, in 



