626 



ECOLOGY 



them their characteristic taste. Even if not poisonous, free oxalic acid 

 certainly is deleterious, since its formation interferes with further cell 

 activity, as does sugar or any other product of metabolism, unless trans- 

 formed into an insoluble substance or removed to other cells. Thus the 

 chief advantage of crystals is in removing oxalic acid from solution. 

 Sometimes it is held that crystals are beneficial in removing calcium 

 from solution, especially in calcareous soils, though this view has not met 

 with general acceptance. It has been held also that raphides by their 

 sharp points protect plants from injury by snails, but the evidence for 

 this theory is inadequate. Still more untenable are the theories that 

 crystals give mechanical support to plants and that 

 they represent accumulations of calcium that later 

 may be utilized. In most cases it is not necessary 

 or even desirable to seek a subsidiary function for 

 the excreted products of plants ; if in certain instances 

 they have such a function, it must be regarded as 

 wholly incidental. 



FIG. 920. A 

 spindle-shaped cys- 

 tolith with warty 

 protuberances from 

 the leaf of Pelli- 

 onia; highly mag- 

 nified. 



Cystoliths. In various Urticales there occur aggregates of 

 calcium carbonate, known as cystoliths, whose rounded rather 

 than angular projections readily distinguish them from crystals. 

 In Pilea and Pellionia they are spindle-shaped and lie free 

 in the cell (fig. 920), while in Ficus they are stalked and 

 mulberry-shaped. When treated with acid, the rounded knobs 

 effervesce, leaving an insoluble, stratified cellulose skeleton. 

 While the role of cystoliths is unknown, it is probable that as 

 waste products they may serve to remove from solution an excess of calcium salts, 

 though some regard them as calcium accumulations that subsequently may be used. 

 Some plants contain warty siliceous bodies somewhat resembling cystoliths. 



Various products of secretion and excretion. Various products, mainly excre- 

 tions, accumulate more in stems than in leaves, and will be considered elsewhere 

 p. 718). The sharp taste of mustards, capers, and nasturtiums is due to an oil, 

 formed by the action of a ferment, myrosin, upon calcium myronate. Many plants 

 contain alkaloids, most of which are violent poisons, as strychnin, airopin, and cocain. 

 Bitter principles are illustrated by the absinthin of wormwood and the aloin of aloes, 

 and poisonous glucosids are represented by digitalin and solanin. Such substances 

 as a class probably are by-products of metabolism, and if, as suggested, they pro- 

 tect against animals, either because poisonous or unpalatable, such protection is 

 purely incidental. Even this incidental use is likely to be overestimated, since 

 plants that are poisonous or disagreeable to man are not necessarily so to all herbiv- 

 orous animals. Furthermore, the organs most likely to be eaten, the leaves, 

 usually contain poisons and unpalatable substances in less amount than do the 

 other organs. 



