IMPERFECT AND RVDIMENTARV BUNDLES. 367 



xylem,— and secondly, those which remain imperfect from the beginning. Both 

 forms are connected step by step with the complete bundles by various intermediate 

 forms, which have already often been mentioned above. 



This especially holds good of those bundles which become imperfect by disap- 

 pearance of the Tracheae. In many herbaceous plants with collateral bundles an 

 intercellular passage appears in place of the primitive vessels when the development 

 of the tissues is complete, as was described at p. 327. 



In a series of other plants, which are submerged or partly submerged aquatics, 

 all the vessels in most of the bundles disappear at once throughout a long part of 

 their course, after they have originated as annular or spiral vessels. In place of the 

 xylem an intercellular canal (filled with water) occurs in the mature bundle, and on 

 its walls the remnants of this thickened membrane may remain preserved. On the 

 other hand, the phloem of the bundles is persistent, and, in many of the cases in 

 question, very well developed. These phenomena present many variations according 

 to the particular cases, and are especially conspicuous in the stems of the Potamo- 

 getons, and of the submerged plants connected with them, which have an axial 

 bundle, or a very simple bundle-system ^ Even in those among these forms in which, 

 as in P. natans, distinct bundles of the leaf-trace and common bundles can be dis- 

 tinguished, the want of vessels on the one hand, and on the other hand the crowded 

 position of the bundles, often gives the whole bundle-system a structure which at the 

 first glance is difficult to decipher, and into this we here have to enter somewhat 

 more minutely. 



The course of the leaf-traces, and of the four cauline bundles in the stem of Pota- 

 mogeton natans and perfoliatus, was described above at p. 272. All the bundles are 

 at their origin collateral, with normal orientation. 



In the node all their parts are persistent, and soon become irregularly united by 

 anastomoses. In the whole internode, on the other hand, the entire xylem disappears 

 from the bundles of the leaf-trace with the beginning of the more intense elonga- 

 tion, and is replaced by an approximately cylindrical narrow intercellular passage, 

 bordered by narrow elongated cells '■'. In P. perfoliatus the same holds good also of 

 the four cauline bundles ; in P. natans, on the other hand, the few (1-3) reticulated and 

 annular tracheae of the latter are usually persistent. The phloem-portions of all the 

 bundles are very well developed and persistent. All the bundles are further closely ap- 

 proximated to one another, being only separated by a few layers of parenchymatous 

 cells containing abundant starch, and traversed by small groups of sclerenchymatous 

 fibres. The bundles are grouped to form an axial strand, rectangular as seen in cross- 

 section, which is marked off from the lacunose cortical parenchyma by an endodermis, 

 which becomes sclerotic subsequently (Fig. 170). Inside this, one bundle faces 

 each longer side of the rectangle ; opposite one of these two sides is a larger bundle, 

 the sympodial one, descending from the second leaf above : facing the other side is 

 a somewhat smaller bundle, the median one of the next higher leaf, belonging to 

 the internode. One of the lateral bundles of this leaf faces each of the shorter sides ; 

 the four cauline bundles face the four angles. A group of sieve-tubes lies at the 



' Compare p. 277, and the literature there cited. 

 '' A. B. Frank, Beitr. z. Pilanzenphysiol. p. 135. 



