252 



BARKS 



been removed, and the exposed part is then the outer layer of bast, 

 which has a rough or rasped appearance and is of a uniform, dark 

 cinnamon-brown colour ; examined under the lens such bark exhibits 

 pale longitudinal strands (sclerenchymatous cells) alternating with 

 darker parenchymatous "tissue (medullary rays). 



The inner surface of the bark is of a cinnamon-brown colour and 

 is finely longitudinally striated or rough, with 

 reticulately anastomosing pale strands, the 

 interstices of which are only partially filled 

 with the brown (parenchymatous) tissue of the 

 medullary rays. 



The fracture is short and granular ; the 

 fractured surface has a reddish grey colour, 

 and usually exhibits numerous tortuous, pale 

 red medullary rays alternating with bast rays 

 containing much sclerenchymatous tissue, the 

 latter projecting beyond the medullary rays, 

 owing to their contracting less on drying. 



The bark has a slight odour of bitter 

 almonds, which becomes much more apparent 

 when it is moistened ; the taste is astringent, 

 aromatic, and bitter, resembling that of bitter 

 almonds. 



The students should observe 



(a) The reddish brown cork with numerous 

 lenticels, 



(b) The irregularly reticulated or fissured 

 inner surface, 



(c) The granular fracture, 



(d) The taste of bitter almonds when chewed. 



FIG. 128. Virginian 

 Prune bark, showing 

 transverse lenticels, 

 land papery cork 

 peeling off. Natural 

 size. 



Constituents. Wild cherry bark yields, when 

 moistened with water, hydrocyanic acid and 

 benzaldehyde. This reaction has been shown 

 by Power and Moore (1909) to be due to 



laevo-mandelonitrile glucoside, C 14 H 17 O 6 N, which is isomeric, but not 

 identical with prulaurasin and sambunigrin (compare p. 37) ; it is 

 hydrolysed by an enzyme also contained in the bark, yielding 

 hydrocyanic acid, benzaldehyde and dextrose. 



Experiments have shown that thin green bark collected in the 

 autumn from trees of moderate size yields most hydrocyanic acid 

 (0*12 to 0'16 per cent.) ; the bark of the root is said to be more active 

 than that from the stem or branches (Stevens, 1896). 



The bark contains also a brown resin, trimethylgallic acid, para- 

 cumaric acid and traces of benzoic acid and volatile oil; a green 

 resin yielding by acid hydrolysis /S-methylaesculetin is also present. 



