^66 PRIMARV ARRANGEMENT OF TISSUES. 



The thin roots of Lycopodium, already described at p. 350, are immediately related 

 to the above. 



Lastly, the roots of Ophioglossum arc to be mentioned here '. In the circular cross- 

 section of the axial bundle it may be seen that the half, which with reference to the parent 

 axis isbasiscopic (lower), is formed of tracheides, which are united together without inter- 

 cellular cavities, and are similar to those of the stem (p. 346). The upper edge of the bundle 

 is formed by a half-ring, usually two layers in thickness, of relatively large, wide sieve-tubes. 

 Between this phloem and the xylem lie some layers, — numbering on the average three, — of 

 delicate prismatic, narrower elements, destitute of starch, the sieve-tube nature of which 

 is doubtful ; as a rule a layer of delicate cells separates the xylem from the endodermis, 

 while the sieve-tubes border immediately on the latter. According to van Tieghem the 

 last statement often holds good also for the two middle tracheides of the lower edge. 

 The development of the tracheides begins at one corner of the segment of the circle, 

 and proceeds from this point round the convex edge, and from this again towards the 

 phloem. The endodermis, like that of the stem, only differs from the other cortical 

 parenchyma in the undulating bands on its radial walls. 



4. Imperfccl and rudimentary Bundle-trunks. 



Sfxt. 1 10, The vascular bundles described above occur in land plants possessing 

 foliage which is rich in chlorophyll, and also in the stems and leaves of parasites 

 which contain no chlorophyll, or only traces of it, as in the Orobanches, Cuscutas, 

 Lennoacese, &c. As indicated at p. 321, they are cceteris paribus, as a rule, all the more 

 developed the greater the development of the leaf-surface. 



Conversely, the development of the vascular bundle-system diminishes in every 

 respect with that of the leaf-surface exposed to the air ; and this is the case firstly 

 with regard to its differentiation into individual bundles and their branches, as is 

 clearly shown by its reduction to an axial strand in the stems of many submerged 

 plants (cf. pp. 277, 321), and by its simplification in the submerged leaves of am- 

 phibious plants (p. 306); and secondly as regards the anatomical differentiation of the 

 individual bundle. In the latter, while the plan of structure remains the same, a 

 diminution of the essential tissue-forms may be recognised, as, for example, the com- 

 parison of the bundles in the stem and root of Ranunculus repens (Figs. 152, 165) with 

 those of R. fluitans (Figs. 153, 163) teaches ; and in fact the diminution is chiefly in 

 the xylem, while the phloem remains the same, or is less reduced. Further, as the 

 characteristic elements, and especially the tracheae, progressively dimmish, deviations 

 from their usual typical arrangement also occur. Again, there may be complete dis- 

 appearance of the tracheal elements, and finally of the sieve-tubes also, so that the 

 entire bundle is replaced by a strand of uniform elongated cells. Lastly, we find the 

 absence even of any rudimentary indication of a vascular bundle, as in the tiny 

 swimming frond of the Wolffias, which is a large-celled mass of parenchyma, covered 

 by an epidermis, which has stomata on the surface in contact with the air '^. 



These cases of imperfectly developed bundles are to be contrasted with the 

 complete ones hitherto considered. They are divided into two main categories, 

 namely, those which originate as complete bundles, and then by disappearance of 

 the xylem become more or less incomplete — being thus bundles with a transitory 



' ^'an Tieg;heni, Rus^ow, I.e. - Hegelmaier, Lemnaccen, p. 31. 



