102 INTRODUCTION TO CYTOLOGY 



maintains that the vacuole-Uke cavities in Triticum are not true sap vac- 

 uoles but masses of an albuminous material which condenses about the 

 plastids with the aid of certain enzymes, the plastids themselves even- 

 tually disappearing. An analogy with starch formation by leucoplasts 

 is pointed out. This conflicts with the Dangeards' claim that aleurone 

 grains represent a stage in the evolution of the vacuolar system. 



Fats and Allied Substances. — Fats and oils are of widespread occur- 

 rence as reserve materials in plants as well as in animals. In plants they 

 are found commonly in seeds, spores, embryos, and meristematic tissues, 

 and occasionally in differentiated vegetative parts. They are chiefly 

 neutral, free fatty acids seldom being present in any considerable amount. 

 They occur emulsified with water and appear in the form of droplets in 

 the cytoplasm, and occasionally in the nucleus also in animals. The 

 assemblage of oily or osmiophilic bodies in the cell is called the ergastome 

 by P. A. Dangeard. 



It appears that materials of this class are ordinarily elaborated directly 

 by the cytoplasm, although in certain plants elaioplasts are evidently 

 concerned (p. 66). Mangenot (19236) attributes oil formation to phgeo- 

 plasts in certain brown algae. The characteristic "oil bodies" in the cells 

 of many liverworts appear to be formed by the fusion of small masses 

 arising in the cytoplasm. ^^ Cytoplasmic origin of essential oils is reported 

 by Popovici (1925) for glandular hairs, and Leeman (1928) describes the 

 passage of such oil into special preformed droplets. It has long been 

 the claim of Guilliermond and his associates that volatile oils have a 

 chondriosomal origin (Guilliermond and Mangenot, 1923). 



Waxes also bear a resemblance to fats in composition and form water- 

 proof coatings on many fruits, stems, and leaves. 



Crystals. — Crystals of many kinds occur in the differentiated tissues 

 of plants. They may lie in the cytoplasm, vacuoles, and occasionally 

 the nucleus. They may be attached to or imbedded in the cell wall; 

 often the cells containing them are considerably modified in size and 

 appearance. They are usually salts of calcium, the oxalate being 

 especially prevalent, and are chiefly by-products. The bundles of 

 needle-shaped crystals of calcium oxalate known as "raphides," which 

 are found in many leaves, arise in the vacuole and come to be surrounded 

 by a layer of mucilage^*' (Fig- 53, L). Definite homogeneous regions 

 in the cytoplasm are thought by Robyns to be concerned in their produc- 

 tion. The spherical crystalline structures known as "druses" (Fig. 

 53, C) are also chiefly calcium oxalate, though they have a central 

 mass of some more complex organic substance. The origin of the druse 

 and its relation to the protoplast have been a subject of controversy, 



15 Pfeffer (1874), Rivett (1918). Gargeanne (1903) held them to arise in sap 

 vacuoles. 



i« E. L. Smith (1923), Robyns (1928). 



