STRUCTURE OF FOSSIL PLANTS 



Fig. 42. — Monostele in which the Pith 

 has invaded all the Tissues as far as the 

 Endodermis, and broken the Wood and 

 Phloem up into Separate Bundles. These 

 are usually called "vascular bundles" in 

 the flowering plants 



are entirely distinct. In the flowering plants the cells 

 of the endodermis are frequentiy poorly characterized, 

 and the pith cells resemble 

 those of the cortical ground 

 tissue, so that the separate 

 groups of wood and bast 

 (usually known as " vascular 

 bundles", in distinction from 

 the "steles" of fig. 40) ap- 

 pear to lie independently in 

 the ground tissue. These 

 strands, however, must not 

 be confused with steles, they 

 are only fragments of the 

 single apparently broken up 

 stele which runs in the stem. 

 The vascular bundle, of 

 all except the Monocotyle- 

 dons, has a potentiality for 



continued growth and expansion which places it far 

 above the stele in value for a plant of long life and con- 

 siderable growth. The cells lying 

 between the wood and the bast, 

 the soft parenchyma cells always 

 accompanying such tissues, retain 

 their vitality and continue to divide 

 with great regularity, and to give 

 rise to a continuous succession of 

 new cells of wood on the one side 

 and bast on the other; see fig. 33, 

 c,b. In this way the primary, dis- 

 tinct vascular bundles are joined 

 by a ring of wood, see fig. 43, to 

 which are added further rings 

 every season, till the mass of wood 



becomes a strong solid shaft. This ever - recurring 



activity of the cambium gives rise to what are known as 



"annual rings" in stems, see fig. 44, in which the wood 



( 122 ) 6 



Fig. 43. — Showing actively- 

 growing Zone c (Cambium) in 

 the Vascular Bundles, and join- 

 fng across the ground tissue be- 

 tween them 



