192 



THE ANATOMY OF WOODY PLANTS 



anatomical evidence or reliable data derived from the study of the 

 remains of plants in the geological strata. 



The stem in this large and important group is distinguished both 

 by the structure of the fibrovascular strands and by their mode of 

 arrangement in a transverse section of the stem (Fig. 142) . The con- 

 tinuous woody cylinder of the older dicotyledons in the herbaceous 

 types more adapted to modern climatic conditions in temperate 



regions has already 

 given place to a con- 

 dition of disconti- 

 nuity. In the 

 monocotyledons the 

 process of disinte- 

 gration of continuity 

 has gone much far- 

 ther than even in the 

 herblike dicotyle- 

 dons. For here the 

 separate strands 

 abandon, under the 

 further influence of 

 the necessities con- 

 nected with the leaves, the original circular arrangement for a 

 scattered disposition through the transverse section of the stem. 

 The high efficiency of the monocotyledons in the elaboration of 

 foodstuffs naturally has correlated with it an ample provision 

 of numerous fibrovascular strands for conducting the assimilates 

 into the stem or axis. This situation brings it about that many 

 traces enter the stem at a node instead of the three or, at most, 

 several traces which pass into the nodal region of the axis in 

 the mass of dicotyledons. This multiplication of the conducting 

 strands brings with it complications of arrangement in the stem, 

 for the number is too large to be accommodated on the periphery 

 as is the rule even among herbaceous dicotyledons. As a con- 

 sequence of the necessities which have thus arisen, the leaf traces 

 are displaced from the margin of the central cylinder into the pith 

 or medulla. It is thus clear that, just as the woody stem has given 



FIG. 142. Stem of Smilax 



