82 ELEMENTARY AGRICULTURAL CHEMISTRY 



The Flowers and Seeds. — In many plants the formation 

 of flowets and seeds is the last act in their life. During 

 flowering the process of true respiration — i.e., the absorption of 

 oxygen and the production of carbon dioxide — takes place more 

 rapidly than at other times during the growth ; indeed, in 

 some cases a distinct rise of temperature has been noted. 

 Respiration occurs during the whole period of a plant's existence, 

 but in daylight it is concealed by the opposite process of 

 assimilation, already described. In biennial plants — e.g.^ turnips 

 and mangolds — the first year of growth is devoted to the 

 formation of a large store of nutritious material, intended to 

 serve during the second year for the formation of flower and 

 seed. 



During the production of seed a concentration of nutritive 

 material, including always albuminoids, phosphates, sulphur, 

 potassium, chlorine, and the other elements essential to plant 

 life, takes place, the stem, leaves and roots being robbed of 

 much of their important constituents. The carbonaceous 

 material in a seed may be either mainly carbohydrates 

 (generally starch), or fats. Most seeds contain mainly one of 

 these classes, but some contain both. 



Conditions affecting- Plant Growth.— Apart from the 



obvious necessity of a proper supply of food and water, the 

 most important factor in growth is undoubtedly temperature. 

 For every plant three important temperatures may be found — 

 the minimum, optimum, and maximum temperatures at which 

 growth occurs. Temperatures below the minimum or above 

 the maximum, though not necessarily fatal to the plant, cause 

 its growth to cease, and in most cases seriously retard its rate 

 of growth for some time after the temperature has risen or 

 fallen above or below the^^e limits. The cardinal temperatures 

 vary considerably with different plants. The minima are 

 usually about 7° or 8° C, the optima about 32° C, and the 

 maxima about 39° to 43° C. 



In all cases the rate of growth increases with a rising 



