208 



Accumula- 

 tion of 

 Starch in 

 the Leaves 

 indicates 

 Root 

 trouble. 



needed, seeing that this is where organic debris chiefly 

 accumulates. Thus from this point of view alone we 

 see how enormously important is the proper aeration of 

 the soil, as also is, of course, the quantity of moisture in the 

 soil, and its temperature, and indeed all factors which may 

 favour or retard the development of these useful living 

 organisms.* 



186. Now if for any of the above 



reasons a plant is unable to obtain its necessary supplies of water 

 and mineral salts from the soil, the carbohydrates manufactured 

 in the leaves cannot be converted into plastic materials 

 and thus be conducted away to the growing tissues which 

 require them, but, owing to the deficiency of water, have to be 

 put aside as starch, and hence an accumulation of starch grains 

 in the leaves is often an indication that we must look to the 

 roots for the cause of a disease. 



ture. 



SECTION IV. INFLUENCES OF THE ATMOSPHERE ON PLANT 



DEVELOPMENT. 



187. For every external factor 



which affects the growth and development of a plant, such 

 as temperature, light, etc., there is, as has already been noted 

 in Part 1 1 1,, a, certain degree of intensity called the optimum 

 which is most favourable to the plant, see page 86. Any 

 Tempera 8 * considerable departure from this limit may cause disease and 

 if carried far enough death. Thus, as regards temperature, 

 the death of plants may be caused either by excessive 

 heat or cold. The power of a plant to withstand extremes 

 of temperature depends chiefly on the species of plant and its 

 stage of development. Most flowering plants are killed by 

 a long exposure to a temperature of 45 C., while some bacteria 

 grow well at a temperature of 70 C. Potato plants may be 

 killed in one night by a temperature of 4 C, while some bacteria 

 can withstand a temperature of 200 C. Khair (Acacia Catechu) 

 and Ber (Zizyphus Jujuba) are frost-resistant, while Aonla 

 (Phyllanthus Emblica) is frost-tender in the climates where 



* In connection with this breaking down of organic material by fungi and 

 bacteria it is interesting to note that this process may not only result in the pro- 

 duction of very valuable plant food materials, but may also convert evil smelling 

 organic substances, such as are contained in sewage, into simpler substances which 

 are absolutely free from odour and inoffensive, a fact which is utilized in the 

 disposal of sewage in towns, the sewage being collected in tanks where it is 

 exposed to the action of various fungi and bacteria. 



