t72 IIOTANY AND PHARMACOGNOSY. 



then miiiicrous oil globules appear iu the protoplasm arouiul it ; 

 later some of the walls of the cell thicken and grow around the 

 crystal, which they finally completely envelop, as in Moraceae. 



PLANT PROTEINS. 



The proteins are nitrogenous compounds, most of which con- 

 tain sulphur and some of which contain phosphorus. Their 

 constitution or the molecular structure of their molecules has not 

 been determined, but they are very large, and are built up of amino- 

 acids, the simplest of which is glycocoll ( amino-acetic acid). 



Apart from the protoplasm found in living cells, the pro- 

 portion of proteins in plants is relatively small, except in seeds, 

 where they serve as nutriment during the germinating period, 

 being made available by the action of proteolytic enzymes. 

 Most of the plant proteins are globulins, and collectively have 

 been termed phyto-globulins. (i) The globulins are insoluble 

 in pure water and in dilute acids, but are soluble in dilute 

 solutions of sodium chloride ( i to 20 per cent. ) , ammonium 

 chloride, sodium sulphate and dilute solution of potassium hydrate, 

 from which solutions they may be precipitated bv dilution, 

 dialysis, or acidification with COo or dilute acids, or by " salting 

 out" by the use of strong or saturated solutions of ammonium 

 sulphate, magnesium sulphate or sodium chloride. (2) The 

 proteins which contain phosphorus are sometimes called phyto- 

 vitellins. as legumin in peas, which contains 0.35 per cent, of 

 phosphorus. A third class of plant proteins, which are alcohol- 

 soluble, are found in cereals, as the gliadin of wheat and rye 

 and the zein of maize. The cohesive and doughing properties 

 of wheat flour are attributed to the association of gliadin and 

 another protein called glutenin. 



Some of the plant proteins occur naturally in the crystalline 

 form, either free in the cytoplasm, as in the potato tuber (Fig. 97, 

 A), or as components of aleurone grains, as in the seeds of 

 Ricinus couununis and P)razil nuts (Figs. 97, D : 122, D). Phyto- 

 globulins in the form of crystals and spheroids have been ob- 

 tained from extracts of flax-seed, hemp-seed. Brazil-nut, castor- 

 oil seeds and others. Protein crystals are, according to Wich- 

 mann, isomorphic, and probably belong to the hexagonal system. 



