MORPHOLOGY 113 



cells of the sieve-tubes, having arisen by longitudinal division 

 from the same mother cell. The companion cells are not so large as 

 the sieve-tubes, and may be distinguished from them by their more 

 abundant protoplasmic contents, and especially by the fact that they 

 retain their nuclei, while the nuclei of the sieve-tubes soon disappear. 

 In Monocotyledons (Figs. 121, 122), and in the Ranunculaceae among 

 the Dicotyledons (Fig. 123), the phloem consists solely of sieve-tubes 

 and companion cells ; in the other Dicotyledons parenchymatous 

 elements are also present, and these are accordingly distinguished 

 as PHLOEM PARENCHYMA ; no companion cells are found in Gymno- 

 sperms and Pteridophytes, - and in addition to sieve-tubes the phloem 

 contains only phloem parenchyma. 



The bundles of the Phanerogams (Gymnosperms and Angiosperms) 

 are generally COLLATERAL in structure, that is, the xylem and phloem 

 are in contact on one side only. In stems the most usual arrange- 

 ment of the two portions of a collateral bundle is that in which the 

 xylem lies nearest the centre ; in leaves the xylem portion lies nearer 

 the upper, and the phloem portion nearer the lower surface. Closely 

 allied to the collateral type is the bicollateral type of bundle. In 

 this the xylem is accompanied by phloem both on the outside and 

 inside. Such bicollateral bundles are characteristic of the Cucur- 

 bitaceae ( 1U ). The xylem and phloem of roots generally form 

 separate strands (Fig. 124 s, v), and the xylem strands are differently 

 oriented ; while in stems the narrow vessels are nearer the centre 

 and the wider nearer the circumference, in roots this order is exactly 

 reversed. 



The "CONCENTRIC" vascular bundles of the Pteridophyta (Fig. 125) 

 contain tracheides (sp), and only in exceptional cases tracheae (sc). 

 The latter are as a rule wanting in Pteridophyta, although this 

 group goes by the name of Vascular Cryptogams. The water-con- 

 ducting elements exhibit scalariform thickenings, only the narrowest 

 having spiral markings (sp) ; they are surrounded by xylem paren- 

 chyma (Ip). Outside this comes a zone of tissue consisting of 

 sieve-tubes (v) and phloem parenchyma (s). 



A number of similar vascular bundles are present in the stem of most Ferns and 

 species of Sclaginella. In Lycopodium they are fused into a single central cylinder. 

 In the stem of Equisetaceae vascular bundles of collateral structure appear. 



The vascular bundles are developed from strands of meristematic 

 tissue which are called PROCAMBIUM STRANDS. Within each strand 

 a zone of tissue commences to divide tangentially, and behaving 

 as a primary meristem produces to both the inside and the outside 

 new cells in radial order. If the whole meristematic tissue of a 

 procambium strand is exhausted in this process, the vascular bundles 

 are said to be CLOSED ; but if any of the meristematic tissue remains 

 in an undifferentiated condition between the xylem and phloem 



I 



