112 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[25. 



y. Proteid Grains, or Aleuron, are granules of various sizes, oval 

 or spherical in form, which occur in seeds, and are of physiological 

 importance in that they are the source from which the embryo is 

 supplied with nitrogenous food when the seed germinates. They 

 consist of a mixture of proteid substances belonging to the globu- 

 lins and the albumoses. They present no indications of structure, 

 and are much larger in oily than in starchy seeds. 



The proteid grain always contains a mass of mineral matter. 

 Most commonly this is a rounded body, the globoid (Fig. 68), con- 

 sisting of double phosphate of lime and magnesia ; less frequently 



there is a crystal, or a 

 cluster of crystals, of 

 calcium oxalate. 



In the large grains of 

 oily seeds it is frequently 

 the case that a portion 

 of the proteid (globulin) 

 of the grain crystal- 

 lises out, constituting 

 the crystalloid ; there are 

 occasionally two or more 

 crystalloids in the grain 

 (Fig. 68). 



The grains are secreted 

 in vacuoles of the cyto- 

 plasm which, when the 

 grains have been dis- 

 solved out, remains as a 

 network (Fig. 68 JD). 



FIG. 70. Part of a section of a grain of wheat 

 Triticum vulgare : p pericarp ; t seed-coat or testa ; 

 internal to which are cells belonging to the endo- 

 sperm; the external layer contains small proteid- 

 grains (ul) but no Ptarch, the more internal cells con- 

 tain sfarch-grains am; n the nucleus. (After Stras- 

 burger : x 240. j 



Proteid crystalloids are also occasionally found, independently of aleuron, in 

 the cells of plants, (e.g. tuber of the Potato ; epidermal cells of leaf of Poly- 

 podium irroides ; some Rhodophyceae). 



The crystalloids (whether free or in proteid grains) differ from ordinary 

 mineral crystals in that, when treated with various reagents, they absorb liquid 

 and swell up. They are for the most part cubical, tetrahedral, or rhomboidal 

 in form. 



8. Mineral. Crystals are frequently found in the cells of plants. 

 They sometimes consist, but in comparatively few cases, of calcium 

 carbonate ; for example, the crystals in the protoplasm of Myxo- 

 mycetes, and the crystalline masses occurring in the cells of the 



