114 



BOTANY 



I'AKT 1 



portions, the bundles are spoken of as OPEN (Fig. 123 c). The 

 Pteridophytes have, almost without exception, closed bundles ; in 



Monocotyledonsalso the bundles 

 are always closed (Fig. 121); 

 Gymnospermsand Dicotyledons 

 (Fig. 123), on the contrary, have 

 open bundles. 



In those portions of plants which 

 are still actively growing in length, 

 the procambium strands remain un- 

 differentiated, except at definite 

 points, where single rows of cells lose 

 their meristematic condition and form 

 narrow, annular, and spiral vessels 

 and sieve-tubes, or sieve-tubes and 

 companion cells ; the structure of all of 

 these is of such a nature as to render 

 their elongation possible. Such prim- 

 ary vascular elements are termed 

 while the correspond- 



Fio. lL'4. Transverse section of central portion of . . , ... 



ing sieve elements are in like manner 

 the root of Aeenu Calamus, m, M'-dnlla ; .--, 6 



xylem; r, phloem ; p, pericycle ; c, endodermis ; designated PROTOFHLOKM. The pro- 

 c, cortex, (x 90.) . toxylem occuj>ies the innermost, the 



protophlocm the outermost side of a 



procambium strand, from which a collateral bundle is eventually formed. After 

 the growth in length of any part of a plant ceases, the differentiation of the pro- 

 cambium strand into a collateral vascular bundle is continued from the inner and 

 outer sides of the strand toward the centre. 



In fully developed vascular bundles the protoxylem and protophloem cease to 

 perform their functions. The protoxylem elements become compressed and 

 ruptured by the tension resulting from the continued vertical growth (a and a', 

 Fig. 122), so that in their stead a lysigenic intercellular space is often formed 

 (Figs. 121, 122). The protophloem elements (cf. Figs. 121, 122) at the same time 

 become disorganised, and their sieve-plates closed by a covering of callus. 



In accordance with the inverted orientation of the xylem, the protoxylem of 

 roots is found on the outer, not on the inner side of the vascular strands (Fig. 124). 



The Terminations of the Vascular Bundles. In leaves, 

 particularly in the foliage leaves of Angiosperms, the vascular bundles 

 become much branched until finally they are reduced to extremely 

 fine strands. In the leaves of Gymnosperms this branching of the 

 bundles does not usually take place, but instead, a single vascular 

 bundle frequently runs throughout the whole length of the leaf. 

 The vascular bundles of the reticulately-veined leaves of Dicotyledons 

 illustrate the most extreme form of branching. 



The minute distribution of the bundles in the leaf-lamina facilitates the regular 

 conduction of water to all parts of the leaf-tissue, and at the same time renders 

 easier the removal of the assimilated products. In the same degree as the ramifica- 



