THE STEM 



195 



what swollen condition. The sieve tubes in the monocotyledons 

 are of a high type and are characterized by terminal walls which are 

 generally horizontal. The lateral abortive sieve plates, known as 

 lattices, present on the side walls of the sieve tubes in woody 

 dicotyledons and many herbaceous ones as well, are usually absent 

 in the group. Both phloem and xylem consequently represent 

 a high condition of organization and indicate an advanced phylo- 

 genetic position for the monocotyle- 

 dons. 



In certain monocotyledonous 

 families cambial activity has been 

 detected in some instances. This 

 phenomenon is quite generally pres- 

 ent in the grasses, as has been pointed 

 out by Chrysler. In the Liliaceae and 

 aroids as well as in the Cyperaceae the 

 presence of a cambium has also been 

 recorded. Fig. 144 shows the pres- 

 ence of secondary growth in the region 

 of the node in Erianthus Ravennae. 



FIG. 144. Bundle of Erianthus 

 Ravennae, showing presence of 

 cambial activity (after Chrysler). 



Frequently the bundles of the stem in monocotyledons are of 

 the closed collateral character, but in some cases, particularly 

 in rootstocks with closely approximated nodes, they present a 

 different type, a concentric condition which differs from that 

 found in the Pteridophyta by the fact that the xylem surrounds 

 the phloem instead of the phloem forming a continuous girdle 

 about the xylem. This modification of the bundle is known as 

 amphivasal to distinguish it from the amphicribral condition charac- 

 teristic of fern allies. A strand of this nature is shown in Fig. 145. 

 The phloem occupies the center of the figure and is surrounded 

 completely by the tissues of the xylem. Bundles of this type are 

 apparently the result of the crowding and fusion of the foliar 

 strands at the nodes, for it is in the nodal region that they are best 

 developed. Where the nodes are remote from one another, the 

 amphivasal condition may entirely disappear in the internode to 

 reappear at the next node. With the approximation of nodes, 

 characteristic of many creeping monocotyledonous rootstocks with 



