48 



ANOMALOUS EXOGENOUS STEMS. 



zilian Sapindacese, such as Paullinia pinnata (fig. 106), Serjania tri- 

 ternata, and Selloviana, there is a central woody mass with from three 

 to ten small secondary ones around it. Each of the masses contains 

 true pith, apparently derived from the cortical cellular tissue, or from 

 the original medullary centre. Gaudichaud and Jussieu state that 

 around these separate collections of pith, there is a medullary sheath 

 and spiral vessels. No annual rings have been detected in the 

 secondary masses, but medullary rays exist, usually in their outer 

 portion (fig. 106). In these anomalous Sapindaceag, the central and 

 lateral woody masses are enclosed in a common bark, with a continu- 

 ous layer of liber. Some have supposed that the lateral masses are 

 undeveloped branches united together under the bark; but Treviranus 

 considers them as connected with the formation of leaves, and as de- 

 pending on a peculiar tendency of the vascular bundles to be devel- 

 oped independently of each other round several centres. 



In some Bignoniacese (fig. 107), the layers of wood are divided in 

 a crucial manner into four wedge-shaped portions by the intervention 

 of plates differing in texture from the ordinary wood of the plant, and 

 probably formed by introversion, or growing inwards of the liber. In 

 some Guayaquil Bignonias, Gaudichaud perceived first four of these 

 plates, next eight, then sixteen, and finally thirty-two. In Aspido- 

 spermum excelsum of Guiana, and in Heteropterys anomala (fig. 108), 



Fig. 105. Horizontal section of stem of Banisteria nigrescens at different ages. 1. Stem 

 presenting four superficial lobes. 2. Showing six deeper lobes, with intermediate divisions. 3. 

 The lobes separated by cellular tissue, the middle one alone having pith and medullary sheath . 

 The points indicate the orifices of porous vessels. 



