DEVELOPMENT OF THE VASCULAR BUNDLES. 399 



In those Phanerogams, in which the pericambium extends over the external corner 

 of the xylem-plates, the same holds good, with the exception of Lycopersicum. In 

 these cases therefore a longitudinal row of the rudimentary roots faces each 

 xylem-plate, an arrangement which is always conspicuous even externally in the 

 case of oligarch roots ; the attachment of the vascular bundles takes place in 

 the form described above at p. 387. Where, on the other hand, the xylem-plates 

 of the roots of Phanerogams directly abut on the endodermis, as is the rule among 

 the Grasses, the rows of rudimentary lateral roots alternate with the xylem-plates, 

 and lie opposite the centre of the phloem-bands, as was stated at p. 387. 



According to van Tieghem's investigations the rhizogenetic longitudinal bands, 

 and the products of their development, have the same position in the case of the 

 Pittosporeae, because a group of oil passages here lies in the pericambium in front 

 of each angle of the xylem, as is to be stated in Chap. XIII. As also described in that 

 chapter, in the case of the Umbelliferae and Araliaceae, e.g. Hedera, an annular row of oil 

 or resin passages likewise lies in the same position, but here a quite similar passage 

 is also present in front of each phloem-group. It is therefore the rule for these 

 plants, apart from individual exceptions described by van Tieghem, I.e. p. 149, that 

 on each side of every angle of the xylem, alternating with the latter and the next 

 phloem-group, there lies a rhizogenetic pericambial band ; thus the number of these 

 bands, and the rows of lateral roots is double that of the xylem-plates, and the 

 attachment of the bundles of the latter takes place as described at p. 387 \ In 

 Lycopersicum, as already stated, the same condition occurs, without any assignable 

 anatomical ground. 



At the conclusion of this chapter it will be convenient to justify some of the opinions 

 here expressed, in opposition to other views. 



In the first place, luith a 'vie^u to simple matters of fact, every distinct strand, which is 

 separate from others of identical or similar structure, and which consists of tracheae or 

 sieve-tubes, has been designated by the name vascular bundle ; to the typical complete 

 strands of this kind, those which remain or become incomplete were appended. This 

 usage is based on the nature of the object, for the fact is established that the two es- 

 sential forms of tissue, tracheae and sieve-tubes, together form these bundles. If the 

 principle maintained in this book, of distinguishing and regarding the systems of tissue, 

 with reference to the forms of tissue which compose them, were carried out with 

 extreme consistency, the phloem of the bundles would no doubt have to be considered 

 separately from their xylem, while the further elements belonging to the bundle would again 

 demand a distinct treatment. It will scarcely however be disputed, that this would not 

 only confuse the description, but also misrepresent the relations which actually exist. 

 If the bundle formed of tracheae and sieve-tubes as its essential constituents, or even 

 each of its two parts, containing one of the essential forms of tissue, be regarded as a 

 whole, it is obvious that the non-equivalent organs, such as parenchymatous cells, &c., 

 which occur in it, must also be reckoned among its elements. If the comprehension of 

 non-equivalent elements is thus once conceded, it may further be extended to those 

 which lie outside, that is, at the periphery of the essential tissues. As soon as this has 

 been done, strict limitation according to the principles above laid down is, it is true, 

 given up ; the actual boundary 'of the vascular bundle becomes conventional, though this 

 conventional limitation may be based upon good grounds derived from other considera- 

 tions, as from that of the primary differentiation of the meristem. It will be so far 

 admissible as, on the one hand, the weight of these reasons, and on the other, advantages 



' See van Tieghem, /. c. ; and Canaux secreteurs, Ann. Sci. Nat. 5 ser. torn. XVI. 



