CONDITIONS WHICH INFLUENCE GROWTH 243 



vigorous common trees, shrubs or herbaceous plants in early summer when 

 they are beginning to grow, and at intervals of two or three days for some 

 time afterwards. 

 Determine the time during which an internode continues to grow in length. 



2. Conditions whicli influence growth. — Only living plants 

 grow and the cells of the growing parts must be in a youthful 

 state. Various external conditions are also necessary for healthy 

 growth, the chief of which are : — (i) a suitable temperature ; 

 (ii) an adequate supply of water ; (iii) appropriate food or food- 

 materials ; (iv) the presence of oxygen, (v) Light although not 

 absolutely essential to growth has a beneficial influence upon it. 



(i) Heat. — It is well known that growth in winter, when the 

 temperature of the surrounding aii and soil is low, goes on 

 very slowly or not at all. As the temperature rises in spring 

 seeds readily germinate and the buds of plants commence to 

 grow ; with the increasing warmth of summer growth becomes 

 more and more energetic. 



By subjecting a plant to a gradually decreasing temperature, a 

 point is at last reached at which growth entirely ceases ; this 

 is described as the minimum temperature for growth. It is 

 not the same for all plants ; the seeds of many common weeds, 

 and mustard and cress germinate, and the fully developed 

 plants continue to grow near freezing-point, while those of the 

 cereals are stopped when the temperature falls to about 5° C. 

 On the other hand the seeds and plants of rnaize and the 

 scarlet-runner bean cease to grow at about 10° C, while the 

 minimum temperature for the germination and growth of the 

 cucumber, melon, and many tropical plants is as high as 19° 

 or 20° C. 



By raising the temperature from the minimum, a point is 

 reached at which growth goes on most rapidly ; this is termed 

 the optimum temperature. By further increasing the temperature 

 beyond the latter point, growth becomes slower and slower 

 until a maximum is attained, at which growth is entirely checked. 



