102 BOTANY: PRINCIPLES AND PROBLEMS 



seed plants; and the herbaceous condition, where the stems are 

 much softer and shorter-lived, has apparently been derived 

 from it in response to climatic changes or for other reasons. 

 In herbaceous species, the amount of fibro-vascular tissue has 

 become proportionally very much less. This may be due simply 

 to a decrease in the activity of the entire cambium, or to the 

 breaking up of the cylinder into separate bundles, but in general 



Fig. 53, Fig. 54. 



Fig. 53. — Transverse section of a one-year-old twig of the sweet gum {Liquid- 

 amhar), showing the continuous fibro-vascular cylinder. 



Fig. 54. — Transverse section of a one-year-old twig of the sycamore (Platanus), 

 showing the fibro-vascular cylinder broken up into segments by the development 

 of wide rays. ■ {Figs. 53 and 54 from Sinnott and Bailey). 



any herbaceous stem is roughly comparable to a one-year-old 

 twig of the particular woody stem-type from which it has been 

 evolved. The herbaceous stem in Fig. 55, with its thin but 

 continuous vascular ring, has probably arisen from some such 

 woody form as is shown in Fig. 53, where the vascular ring is 

 similarly continuous and homogeneous. The stem in Fig. 56, 

 however, in which the cylinder has been broken into distinct 

 and completely separate bundles, is quite different in type and 

 has probably arisen from a woody stem somewhat resembling 

 that in Fig. 54, where the vascular ring is divided into segments 

 by the development of very wide rays. Cambial activity is 

 usually weaker opposite these rays than opposite the woody 

 segments of the cylinder, and in the stouter herbs of this type, 



