1883.] in the Monocotyledonous plant. 397 



scutellum. The arrangement of the fibro vascular bundles in the 

 root is on the radial type, as in Phoenix Dactylifera, only there are 

 many more bundles of Xylem and Phloem iD the root of Zea Mais 

 than in that of Phoenix Dactylifera. On the outside of the fibro- 

 vascular cylinder in older roots there is an endodermis of one or 

 two layers of sclerenchymatous cells. As we approach the node B, 

 passing upwards, the bundles become irregular in form and show 

 signs of rotation as they enter the anastomosis of the node, but it is 

 impossible to follow the course of each bundle through the node. 

 Between the nodes A and B the arrangement of the bundles is 

 irregular. In Fig. vm., which is a drawing of a section cut between 

 the nodes A and B, spiral vessels are seen at S irregularly placed 

 inside a more or less continuous ring of Xylem, often opposite the 

 Phloem bundles Ph, which are external to the ring of Xylem. This 

 arrangement continues as far as the node A, where the bundles 

 again undergo an anastomosis, and above this we find the typical 

 structure of the grass stem, viz. irregularly scattered fibrovascular 

 bundles arranged on the collateral type and not enclosed by an 

 endodermis. 



A comparison between the passage from root to stem in 

 Monocotyledons and Dicotyledons may now be made. 



In the procambium of the root the protoxylem or spiral vessels, 

 and the protophloem or bast fibres are first differentiated, the 

 differentiation in each bundle proceeding from without inwards 

 and thus the separate Xylem and Phloem bundles are produced. 

 In the stem each bundle consists of Xylem and Phloem. The 

 protoxylem is first differentiated at the most internal part of each 

 bundle, and the differentiation proceeds from within outwards, 

 while the protophloem is first differentiated at the most external 

 part of each bundle, and the differentiation proceeds from without 

 inwards. 



Thus in the stem the protoxylem is the most internal part of 

 each collateral bundle, and in the root the most external part of 

 each bundle of Xylem, and in passing from the root to the stem 

 there is necessarily a rotation of the secondary Xylem round the 

 protoxylem, or a line drawn through the protoxylem in the direc- 

 tion of the differentiation of the constituents of each Xylem bundle 

 would in passing from root to stem turn through two right angles. 

 Since in the root each bundle consists of Xylem or Phloem only, 

 and in the stem of both Xylem and Phloem, it follows that in the 

 transition from stem to root the Xylem and Phloem of each fibro- 

 vascular bundle of the stem must separate from each other. The 

 Phloem bundles unite in pairs, the Xylem bundles rotate and 

 unite in the centre of the root. 



In the Dicotyledon the transformation from the arrangement of 

 the bundles in the stem to that of the root generally takes place in 



•2b— 2 



