SPECIAL TYPES OF NUTRITION 243 



allo formed as the result of the activity of special plastids termed 

 elaioplasts. Crystals of various mineral salts, and the varied 

 contents of laticiferous tissue including resin, also may be included 

 among the reserve materials, and even the poisonous acids and 

 alkaloids may sometimes be regarded as serving similar pur- 

 poses. 1 



310. Determination of the Storage Substances in a Plant. Se- 

 cure several specimens of Helianthus, Solatium, Avena, Pisum or 

 Phascolus and make a complete examination to identify and locate 

 the different substances stored as reserve food in various organs. 



311. Formation of Storage Organs and Deposition of Reserve 

 Material. Cultivate a number of specimens of Solatium from cut- 

 tings of tubers, and follow the development of the new storage tuber 

 formed at the base of the new stems. Note the enlargement of 

 the tissues and the accumulation of the reserve material (See 

 formative effect of light). 



312. Special Types of Nutrition. The method of nutrition by 

 which mineral salts in simple combinations are taken up from the 

 substratum, and carbon dioxide is absorbed from the air is the 

 prevailing one in the vegetable kingdom. The essential feature 

 of this method is the absorption of energy direct from solar radi- 

 ations by means of a specially developed chlorophyl screen. 

 Practically all organic substances have been constructed by means 

 of the energy thus derived. Plants as a group build up many 

 times as much material as they use in growth and development, 

 and the death of the successive generations of individuals adds to 

 the store of organic matter on the surface of the earth, which is 

 constantly undergoing decay and decomposition, forming humus 

 in the process. The remains of plants contain all of the sub- 

 stances formed in living material, but in various stages of disinte- 

 gration into simpler compounds. The compounds in the humus 

 contain much more chemical energy than the simple salts forming 

 the major portion of the soil products used by green plants, but 

 their complexity is such that they do not easily pass the plasmatic 



1 Pfeffer. Physiology of Plants, I : 604. 1900. 



