134 



LECTURE VIII. 



are only filled with water in special cases, still remains doubtful. On the other 

 hand, we may regard the phloem, in the main, as the form of tissue in which 

 albuminous substances containing nitrogen are produced, and by which they are 

 distributed for long distances in the organs : the elements of the phloem may also 

 serve for the transport of products of assimilation. The vascular bundles of simpler 

 structure, especially in water-plants, are probably confined to these functions ; while 

 in cases where, with the increase of the assimilating foliage, a more rapid bringing 

 of water and carrying away of products of assimilation are necessary, greater com- 

 plexities in structure occur. In land-plants especially the tendency prevails to 



produce elastic fibres within the vas- 

 cular bundle itself; that is, wood- 

 cells in the xylem, and true bast- 

 fibres in the phloem. In other cases 

 the vascular bundle is enveloped, on 

 one side or all round, with such 

 elastic fibres : this contributes to the 

 solidity of the organs, especially of 

 the shoot-axes and leaves, and as 

 I hope to show later, the conduction 

 of water is facilitated at the same 

 time. 



The arrangement of the xylem 

 and phloem in a vascular bundle 

 presents many noteworthy differences, 

 according to the organ and the class 

 of plant : it is not possible however, 

 apart from details, to establish a re- 

 lation between them and definite 

 physiological principles. It may 

 therefore suffice, with the help of the 

 accompanying figures, simply to point 

 out here a few of the most important 

 differences in this connection. Thus, 

 we find in the Ferns, as Fig. 139 

 shows, the xylem enveloped all 

 round by a more or less thick layer 

 of phloem. In the Monocotyledons, 

 on the other hand, and particularly in the Grasses, the latter forms a strand, which 

 is accompanied on its inner side (i. e. the side turned towards the axis of growth of 

 the organ) by the xylem, which also envelopes it more or less laterally (Fig 137). 

 Fig. 138 gives an approximate representation of the ordinary structure of the vas- 

 cular bundle of the Dicotyledons. With respect to details I may refer to the 

 explanations of the figures, with the remark that in the monocotyledonous bundles 

 (Fig. 137) the thick-walled cells, forming a sheath surrounding the whole proper 

 vascular bundle, do not belong to the bundle itself, but represent one of the 

 frequently occurring arrangements to ensure solidity appertaining to the fundamental 



Fig. 139.— Portion of the transverse section of a large vascular bundle 

 from the stem of Pleris aquilina, with some of the surrounding paren- 

 chyma {P): the latter is filled (in winter) with starch, s spiral vessel in 

 one focus of the elliptical transverse section : this is surrounded by thin- 

 walled wood-cells, containing starch. £, g scalariform vessels and sp 

 sieve tubes. Between these and the xylem is a layer of cells bearing 

 starch in winter, b bast cells with thick soft walls; sg vascular bundle 

 sheath. Between sg and i is a layer of cells containing starch. 



