LEAVES 



625 



The influence of external factors upon gland development and secretion. Almost 

 nothing is known concerning the influence of external factors upon gland develop- 

 ment or secretion. In some cases glandular and " protective " hairs arise from 

 similar primordia (as in Verbena, fig. 914), and it has been claimed that the condi- 

 tions to which such primordia are subjected determine the kind of hair that develops ; 

 for example, in Spiraea, primordia that are exposed to severe conditions (especially 

 to cold) develop into long, thick-walled protective hairs, while other primordia 

 develop into glands. Similarly, resins and oils are thought to be secreted more 

 abundantly, even in the same species (as in Rumex Acetosella and in Primula obco- 

 nica), in xerophytic than in other situations. Slime glands are better developed in 

 the water leaves than in the air leaves of Myriophyllum proserpinacoides. 



Crystals and cystoliths. Crystals. Calcium oxalate is a very com- 

 mon substance in plants, occurring in the form of needlje-shaped crystals 

 (raphides, fig. 918), or crystal aggregates (fig. 919), 

 or, much more rarely, in the form of isolated 

 crystals with more or less equal axes. Crystals 

 commonly are confined to 

 parenchymatous cells, which 

 often are arranged in longi- 

 tudinal rows close to the con- 

 ductive bundles. Raphides 

 usually are grouped in bundles, 

 the individual crystals being 

 oriented in a common direc- 

 tion; they are particularly 

 abundant in monocotyls with 

 slimy cell sap, while crystal 

 aggregates are perhaps more 

 common in the stems of di- 

 cotyls. Crystals sometimes 



FIG. 918. A cor- 

 tical cell from the stem 

 of the wandering Jew 

 (Zebrina pendula), 



showing a group of occur in veryminute form, con- 

 stituting the so-called crystal 

 sand. 



Calcium oxalate crystals are 

 undoubted excreta, represent- 



needle-like crystals (ra- 

 phides) of calcium ox- 

 alate; note the paral- 

 lelism of the crystals 

 to one another and to 



ZSLAAS '* by-products of metabo- 

 magnified. lism. Oxalic acid in the free 



state, existing as a solute in 

 the cell sap, is believed to be poisonous, especially if present in large 

 amount, though in the various sorrels it is abundant enough to give 



FIG. 9 19. A longi- 

 tudinal section of the 

 cortex of a petiole of 

 the hop tree (Ptelea tri- 

 foliata), showing longi- 

 tudinal rows of short 

 cells, each with a com- 

 pound crystal of cal 

 cium oxalate; highly 

 magnified. 



