114 



PLANT ORGANS OR PARTS OF PLANTS. 



needles of chrysophanic acid (?), parenchyma, vessels, 

 and fibres. 



The crystals form the most conspicuous feature in the 

 powder; they are large and numerous, sometimes con- 

 stituting 30 per cent, of the whole ; they are of the rosette 

 type and vary widely in size, measuring up to 100 microns. 

 Some tabular octahedral forms are also observed. 







0^0 e^ ^ 



Fig. 23. — Rhubarb in Powder. 

 S, Starch; C, crystals; P, parenchyma; A, chrysophanic acid, 

 crystals (?); Pt, parenchyma, twisted and contorted; V, pitted and 

 reticular vessel fragments; Va, fragments of large vessels; F, fibres. 



The starch grains are for the most part simple, though 

 compound grains are not rare. They are spherical to 

 slightly angular and average about 20 to 30 microns. Their 

 polarization cross is conspicuous; the arms are at right 

 angles and attenuate in the centre. Peculiar acicular clus- 



