ANOMALOUS EXOGENOUS STEMS. 



61 



I 



vascular bundles, b b b b h, form zones, which are each the produce of 

 several years' growth, and 



are separated by layers, ^ ^ ,a / ? ' !,. 



mill, which may be con- ,7 ^ ' 



sidered as representing dif- 



ferent zones of liber. 



In some of the Meni- 

 spermum tribe, the sepa- 

 rating layers are of a cellular 

 and not of a fibrous nature. 

 In Banisteria nigrescens 

 (fig. 123), the young stem 

 (1) presents a four-lobed 

 surface ; the lobes become 

 more evident (2) ; and ul- 



Fig. 122. 



timately (3) the stem is divided into a number of separate portions, the 

 central one of which alone exhibits pith and medullary rays. The 

 portions are separated by interposed cortical layers. 



Many of the Malpighiacese, Sapindaceae, and Bignoniacese of Brazil, 



exhibit stems in which the woody layers are arranged in a very irre- 



1 



Fig. 123. 



gular manner. In the stem of Calycanthus floridus, and of some 



Fig. 122. —Horizontal section of stem of Gnetnm. m. Pith, e m, Medullary sheath. 

 & 6 6 6 6, Woody bundles forming seven concentric zones, each of which is the produce of 

 several years. II I III, Fibres of liber forming interposed circles, equal in number to the 

 woody zones. Fig. 123. Horizontal section of stem of Banisteria nigrescens at different 

 ages. 1. Stem presenting four superficial lobes. 2. Six more marked lobes, with inter- 

 mediate divisions. 3. The lobes separated by cellular tissue, the middle one alone having 

 pith and medullary sheath. The dots indicate the orifices of pitted vessels. 



