PALM-TYPE OF VASCULAR SYSTEM 383 



also remarked upon the close resemblance between the vascular reticu- 

 lum in the cortex of many succulents (Salicornia, species of Mesi mbryan- 

 f/irmum, CACTACEAE) and the vascular system of an ordinary leaf. 

 Heinricher, finally, has demonstrated the presence of a cortical vascular 

 system in association with axial photosynthetic tissue in the genus 

 Gentaurea. 



4. The majority of Monocotyledons are provided with the form of 

 vascular system termed the Palm-type by Von Mohl, which is charac- 

 terised by the following features. In the first place, all the bundles 

 are common, and as a rule a large number of bundles (up to several 

 hundreds) enter the stem from the sheathing base of each leaf. 

 Secondly, instead of all the strands of each leaf-trace running at the 

 same depth in the stem, the median bundles always pass further 

 inwards than the lateral ones. Thirdly, every bundle that penetrates to 

 any considerable depth finally turns outwards again, thereafter pursuing 

 an obliquely radial and downward course towards the periphery ; only 

 the lateral bundles descend almost perpendicularly. The majority of 

 the strands are not only radially inclined, but also deviate tangentially 

 and at the same time become spirally twisted, both features being most 

 pronounced in short internodes. Lastly, every bundle descends as an 

 independent strand through a large number of internodes, finally uniting 

 near the periphery with a strand which pertains to a leaf inserted much 

 further down the stem. It is on account of this characteristic behaviour 

 that the vascular bundles in the transverse section of a Monocotyle- 

 donous stem appear to be arranged in a " scattered " or " irregular " 

 fashion, with a tendency to become aggregated near the periphery. 



In attempting to explain the physiological significance of the Palm- 

 type of vascular system, we must first of all note that the equalisation 

 of the supply of water and nutrient salts to the several leaves, and the 

 uniform distribution of the synthetic products over the entire cross- 

 section of the stem, are in this case sufficiently ensured by the inclusion 

 of a very large number of strands in each leaf-trace, and by the fact 

 that these strands enter the stem over a large portion of its circum- 

 ference. The insertion of the lower ends of the leaf-trace strands upon 

 bundles which enter the stem further down, is therefore a matter of 

 less urgency here than it is where the Dicotyledonous type of 

 arrangement prevails, and is consequently a feature of minor importance 

 in stems with Palm -structure. The outward trend of the leaf-trace 

 strands in the latter part of their course is, in all probability, mainly 

 determined by mechanical considerations. The vascular bundles are 

 generally provided with partial or complete fibrous sheaths ; in 

 accordance with the rules of inflexible construction, however, these 

 mechanical strands tend to approach the periphery, and, in so doing, as 



