COMPOSITION OF FIBRO-VASCULAR BUNDLES OF PLANTS. 393 



the parenchymatous, with this single exception, that Nageli 

 speaks of parenchyma, other than the soft hast, as existing 

 in the phloem, whereas Sachs makes no mention of it. We 

 have yet to notice what is the current view of the nature of 

 the mass of tissue occurring in the bundles of Monocotyle- 

 dons termed by Nageli cambiform, and which he described 

 as separating the xylem from the phloem. This cambiform 

 tissue is now regarded as being the equivalent of the soft 

 bast of the bundles of Dicotyledonous plants. 



The elements comprising the fibro- vascular bundles of the 

 vascular Cryptogams may also be classified under the heads 

 of vascular tissue, prosenchyma, and parenchyma; but their 

 occurrence is not so constant as in the Phanerogams. 



In the stem of Equisetum the xylem consists of parenchyma 

 and of vessels, and the phloem of parenchyma, bast fibres, 

 and vessels (sieve tubes). The bundles of the stem of Lyco- 

 podium differ in histological structure from those of Equi- 

 setum, in that here the xylem consists only of vessels which 

 are no longer separated by parenchymatous cells (geleitzellen 

 of Russovv), whereas the phloem presents three characteristic 

 elements. The elements present in the bundles of Pteris 

 aquilina are the same as those occurring in Equisetum, and 

 this is true also of Marsilea, but in this case the parenchy- 

 matous cells of the xylem (geleitzellen) are very few. 



The grounds upon which certain of the elements of fibro- 

 vascular bundles are referred to the phloem or to the xylem 

 deserve consideration. In the case of Dicotyledons it is 

 evident that all those elements of the bundle which are 

 external to the cambium layer belong to the phloem, whereas 

 those which are internal to it belong to the xylem, and 

 Schwendener is, perhaps, right in saying^ that Nageli had 

 this relation of the inner and outer parts of the bundle to 

 the cambium in view when he first suggested the use of the 

 terms. But the distinction of these two parts of a bundle 

 from one another is by no means so simple when the bundle 

 belongs either to a Monocotyledonous or to a Cryptogamous 

 plant. In such a case a decision can only be arrived at by 

 detecting characteristic histological elements, such as the 

 vessels, in different parts of the bundle. The vessels of the 

 phloem and those of the xylem, however, are generally sepa- 

 rated by parenchymatous cells, and these present no such 

 difference as would lead to any definition of one part of the 

 bundle from the other. Russow^ has endeavoured to remove 

 this difficulty by regarding the parenchyma of both phloem 



' Loc. cit., p. 4. 



2 ' Jjclraclituugcn,' p. 17. 



