144 



GENERAL BIOLOGY 



bundles, and even these are very weak in a prostrate 

 herb like the chickweed. The bundles are arranged 

 in a ring around a central core of parenchyma (the 



Fig. 91. Diagrams of stem sections (exogens). a, cross-section of 

 chickweed stem, the inner circle representing the cambium ring, 

 the two radial lines indicating the portion enlarged in b', 

 e, epidermis; /t, hair; c, cambium separating between p, phloem 

 and w, woody portions of bundles; v, spiral vessels in the 

 woody portion; x, pith and y, common parenchyma of bark; c, 

 segment of a sunflower stem; p, parenchyma; 6, bast fibres; s, 

 sieve tube; c, cambium; g, vessels, pitted and spiral; h, wood 

 fibres, (from Wettstein) ; d, one year, and e, four year old woody 

 stems, illustrating the increase of vascular bundles. 



