120 BOTANY. 



148. Nageli extended this classification of the tissues to 

 the fibro-vascular bundles of Monocotyledons, and subse- 

 quently it has been still further extended so as to include all 

 kinds of fibro-vascular bundles. In every case the tracheary 

 portion is the essential, or most constant, characteristic of 

 the xylem, as the sieve tissue is of the phloem. 



These terms are valuable when used in reference to the 

 fibro-vascular bundles of the stems of Phanerogams ; they 

 may also be valuable, if properly used and understood, when 

 applied to other forms of the fibro-vascular bundle. The 

 xylem portions of the stem bundles of different plants 

 among the Phanerogams are homologous parts of the tissue 

 systems the bundles ; but when the term xylem is applied 

 to certain parts of two dissimilar bundles e.g., of Ricinus 

 (Fig. 106) and Lycopodium (Fig. 101) no homology of parts 

 should be understood. ' The tissues themselves, in some 

 cases of dissimilar bundles, may be homologous, but they are 

 homologous tissues, and not homologous parts of a system 

 of tissues.* When, therefore, these terms are used in the 

 present work, it must be borne in mind that they do not 

 necessarily convey the idea of homology of parts. 



149. De Bary's f recent structural classification of fibro- 

 vascular bundles is useful in designating their general plan. 

 He includes all forms under three kinds, viz., (1) the Col- v 

 lateral bundle, which has one mass of xylem by the side of 

 a single mass of phloem ; this is the form of all bundles of 

 the stems of Equise'tum, and of the stems and leaves of Pha- 

 nerogams I (Figs. 99, 102, 103, 106, 107) ; (2) the ConcentricX 



* This point, which is an important one, may be made clearer by an 

 illustration from zoolojry. The nervous tissue of one animal is the 

 homologue of that found in any other, but the nervous system of one 

 may or may not be the homologue of the other. The nervous system 

 of the bee, for example, is not the homoloprue, but the analogue, of 

 that of the ox ; it is, however, the homologrue of the nervous system 

 of the lobster. The brain of the ox and the brain of the bee are not 

 homologues as parts of a system, but they are homologues as tissues. 



\ " Vergleichende Anatomic," etc., p. 331, et seq. 



| In the Cucurbitaceae and some other orders there is a mass of sieve 

 tissue on the inner side of the xylem, so that the latter is between two 



