MICROSCOPIC STRUCTURE 127 



noticeable in the arrangement of its vascular 

 bundles in the shape of separable wedges, and 

 in the absence of concentric rings or zones. 

 The wood of this plant differs in appearance 

 from that of exogenous trees or shrubs, as it 

 consists of radiating plates of wood surrounding 

 a pith and encircled by bark. 



The section shows at the centre the pith 

 containing cells, and around this a belt of 

 isolated vascular bundles in which the wood is 

 the darker portion, pierced with holes, which 

 latter are sections of vessels. 



The plants of this genus are for the most 

 part shrubs, generally climbing round the 

 branches of trees. They abound in tropical 

 South America. 



The flowers of some of the kinds are remark- 

 able for the oddity of their forms and for their 

 large size. Humboldt mentions one growing 

 on the banks of the Magdalena as having 

 blossoms, measuring 4 feet in circumference, 

 which the Indian children sportively draw on 

 their heads as caps. 



Several kinds of Aristolochia are cultivated 



