STRUCTURE OF COLLATERAL BUNDLES. 32 1 



plants, especially Monocotyledons, and water and bog-plants, have only a group of a 

 few (3-6) tracheae, and a phloem portion limited to about 20 or fewer elements; in 

 the stems and leaf-stalks of many Aroideae the number of the tracheae falls in many 

 bundles to 2 and i^; on the other hand, the thicker bundles of Monocotyledonous 

 plants, and above all the thick bundles of the Ferns above mentioned, and of the 

 leaves of Dicotyledonous land-plants, present very high figures. It is an obvious 

 a priori conjecture that a definite ratio exists between the size of the bundles, 

 especially of their vascular parts, and their number, and that both are definitely 

 related to the extent of the transpiring and assimilating leaf-surface, as well as to the 

 vigour of the root system and the arrangement of the roots. A number of facts, 

 which have partly been stated here, partly in Sects. 61-71, and are still to be 

 considered in Chap. XIV, point to such relations. The comparison of nearly-related 

 species inhabiting the water and the land respectively, demonstrates among the 

 former a considerable diminution in the development of the bundles (cf. e. g. Figs. 

 153 and 154), which may extend to the entire disappearance of the xylem. A 

 sufficient basis, however, for the attainment of general results, available for more than 

 individual phenomena, is still wanting, so that we can here only point out these 

 manifest relations without entering into them more minutely. 



The structure of the two parts of the bundle, in so far as it is brought about by 

 the juxtaposition of the different sorts of tissue, and by their peculiarities, as described 

 in former chapters, will in the case of the Osmundacese and Marattiaceae be 

 mentioned further on, under the head of concentric bundles in Ferns (Sect. 106). In 

 the other collateral bundles, especially those of the Phanerogams when they consist 

 of more than one or two elements, — 



(a) The xylem is built up of tracheae and (parenchymatous) cells. At its inner 

 edge lie a few narrow, spirally or annularly thickened tracheae, which are the first 

 products of the differentiation of the tissues, and are thus to be called primitive 

 elements (Protoxylem of Russow). For the reasons given at p. 157 it is especially 

 these primitive elements in which, in the mature plants, the spiral threads are 

 steeply wound or quite distorted, and the rings widely and often irregularly separated 

 from one another. The primitive elements themselves are not uncommonly com- 

 pressed by the expansion of their neighbours, and here and there manifestly 

 destroyed. In the Coniferse, Equiseta, and Ophioglosseae, the primitive elements are 

 tracheides ; the same may be the case in the plants or parts of plants mentioned 

 above at p. 165 as wholly destitute of vessels. In the other cases they are called 

 vessels, and in most instances no doubt rightly so, although just as in the case 

 of the primitive tracheae, few very accurate investigations of this really not very 

 essential distinction have been undertaken. Outside the primitive elements wider 

 tracheae follow, which are tracheides or vessels according to the individual cases 

 mentioned in Chap. IV, and especially in Sect. 40 ^. Their development takes place 

 successively, advancing from the inner edge of the bundle outwards, and as a rule at 

 a time when the elongation of the entire part to which they belong is nearly at an 

 end. The thickenings on their walls therefore have a successively denser arrange- 

 ment : dense spiral and annular tracheae, then reticulated and pitted tracheae follow 



' Compare van Tieghem, Struct, des Aroidees, I.e. ^ Compare details in Caspary, I.e. 



Y 



