NUTRITION. 



151 



useless materials are gotten rid of; or they may be excreted 

 directly. They may be called waste materials. 



211. Waste materials. — Among the most important are the carbon 

 acids, such as oxalic, malic, etc., the tannins, the resins, the gums, the 

 volatile oils, and the alkaloids. These substances are either by-products 

 of photosynthesis, or they arise in the course of the assimilation of foods. 

 Oxalic acid is usually gotten rid of by being combined with calcium to 

 form calcium oxalate, which crystallizes either in the form of squarish 

 crystals or as long needles (fig. 115). The resins, usually dissolved in an 

 oil, are generally excreted into special intercellular spaces. Volatile oils, 

 to which most odors of plants are due, are secreted by glandular hairs 

 (£•, fig. 74) ; or are formed in the epidermis itself, as in flowers ; or are 



Fig. 115. Fig 116. 



Fig. 115.— Crystals found in plants. I, calcium carbonate; II-IV. calcium oxalate; 



11, octahedron with blunt ends ; III, compound crystals from the nectary of a mallow ; 



IV, rt, b, needle crystals (laphides) from leaf of fuchsia. All highly magnified.— 



After Behrens. 

 Fig. 116.— Section through oil-receptacles in rind of orange. The upper figure shows 



the structure at the beginning of the disorganization ot the oil-producing cells ; the 



lower, the final condition, with two drops of oil occupying the cavity. Moderately 



magnified.— After Tschirch. 



produced in chambers near the surface, the cells which produce the oil 

 being disorganized to form the cavity in which the drops lie (fig. 116). 

 Many of the alkaloids, such as quinin, morphin, strychnin, nicotin, etc., 



