SECT. I MORPHOLOGY 137 



whereby the successively formed procambial strands are placed at 

 different depths (^^*). A common arrangement of the bundles in 

 monocotyledonous stems is that of the so-called Palm type, in which 

 each leaf-trace consists of the numerous bundles which pass singly into 

 the stem from the broad leaf-base. The median bundle penetrates to 

 the middle of the stem. The depth to which the lateral bundles 

 penetrate lessens with their remoteness from the median bundle. In 

 their descending course the bundles gradually curve outwards, and 

 finally join other bundles near the periphery of the stem. The 

 number of internodes, therefore, through which a bundle passes 

 before coalescence, is variable ; the median bundle, however, continues 

 distinct for the longest distance. The deeper penetration and 

 greater length of the median bundle (m) become apparent in a 

 median longitudinal section of such a stem (Fig. 146). The numerous 

 bundles entering the stem from axillary shoots pursue a similarly 

 curved course to those entering from leaves, and end by fusing with 

 peripheral bundles. 



In addition to the leaf-trace bundles or COMMON bundles Avhich 

 are common to both leaf and stem, there are others, called CAULINE 

 BUNDLES, which belong solely to the stem, and again others, foliar 

 BUNDLES, which on entering the stem from the leaf, at once coalesce 

 with other bundles and have no independent existence in the stem. 

 The bundles of the Pteindophytes are continued as cauline bundles in 

 the stem, and those from the leaves join on to the bundles of the stem. 



Tlie stems of some Dicotyledons {Begonia, Aralia) possess cauline bundles in 

 addition to leaf-traces. The cauline bundles are situated in the pith within the 

 ring of leaf-trace bundles in the internode, and at the nodes are connected by 

 branches with the leaf-trace bundles. 



In the central cylinder of roots, the radially arranged strands of 

 xylem and phloem pursue a straight course parallel to one another. 

 The xylem and phloem strands of lateral roots are inserted upon the 

 corresponding strands of the parent root. Those of adventitious 

 roots are attached to the corresponding tissues of the stem or root 

 from which the adventitious root arises. The strands of xylem and 

 phloem of the main I'oot of a phanerogamic seedling (p. 168) are con- 

 tinuous with the leaf-trace bundles in the hypocotyl. 



Increase in Thickness by Continued Enlargement of the Primary 

 Meristem. — The growing points of those Monocotyledons which 

 have large stems (Palms, Pandanaceae, some Liliflorae) exhibit this 

 feature. The growing point thus attains a considerable thickness, and 

 the stem has from an early period its definite circumference. The 

 increase in number of cells takes place in a zone lying at the 

 periphery of the growing point i^'^-'). 



Growth in Thickness of the Stem by means of the Continued 

 Enlargement of the Fundamental Tissue. — This is often exhibited 



