i8o 



THE ANATOMY OF WOODY PLANTS 



tissues are imagined to have been originally quite distinct from one 

 another and to have been linked up by the formation of woody 

 commissures organized entirely of secondary xylem and laid down 

 by the activity of the so-called interfascicular cambium. The 

 narrower and more depressed segments are supposed to owe their 

 peculiarities to their late and entirely secondary origin. The five 

 outstanding segments of wood in the oak and similar forms have 



been accordingly dubbed 

 the fascicular wood, and 

 the five intervening de- 

 pressed segments the 

 interfascicular wood. 

 An unfortunate situa- 

 tion often encountered 

 by this hypothesis is the 

 fact that primary wood 

 is frequently as well 

 developed on the inner 

 surface of the depressed 

 segments as on that of 

 the outstanding ones. 

 The depression, as has 

 been pointed out above, 

 is susceptible of an 

 entirely different 

 explanation namely, as 



the result of the local inhibiting influence of approximated broad 

 rays on the rate of growth of the annual ring. In effect, moreover, 

 the hypothesis of Sanio and Sachs derives woody from herbaceous 

 forms, a conclusion entirely at variance with the paleontological his- 

 tory of plants. It is clear that the woody forms have preceded 

 herbaceous ones in all the main series of vascular plants. A single 

 illustration will serve in the present connection. Our somewhat 

 herbaceous existing lycopods and Equiseta are certainly known to 

 have come from ancestral forms which possessed so conspicuously 

 the arboreal and perennial habit that for many years a controversy 

 raged as to their affinities. The majority of paleobotanists for a 



FIG. 133. Transverse section of a twig of the 

 velvet oak (Qiiercus veliitina) showing five pairs of 

 foliar rays. 



