SECRETORY AND EXCRETORY SYSTEMS. 



145 



elements. In young roots and stems the 

 individual bundles lie distinct and separ- 

 ate. As the growth of the root or stem 

 proceeds they gradually coalesce, the 

 cambium zone becomes complete and the 

 bundles remain separated only by thin 

 radiating lines of tissue termed the Med- 

 ullary Rays. By virtue of the meriste- 

 matic power of the Cambium, the Xylem 

 is constantly growing centripetally, 

 while the Phloem grows ceutrifugally. 



Fiff. 61.— Cross section of root of Althaea ofa- 

 cinalis. c. cambium; m. medullary rays. sch. mu- 

 cilage cells, h, bast fibres, hp. woody parenchyma 

 filled with starch, sb. sieve portions, lb, libriform. 

 (Tschirch). 



The bundles are therefore capable of in- 

 definite extension, and are consequently 

 called Open Collateral bundles. 



b. The Closed Collateral bundle is 

 characteristic of the Monocotyledons. 

 The elements are grouped together with- 

 out any cambium, and are generally sur- 

 rounded as a whole by a circle of schler- 

 enchymatic fibres which checks the la- 

 teral growth of the bundle. To this ar- 

 rnngement is due the fact that Mono- 

 cotyledons after forming their bundles 

 do not grow in thickness but only in 



height. (Fig. 59.) 



c. The Bi-Collateral Bundle has two 

 strands of Thloem, one on each side of 

 the Xylem, It is found in the Cucurbi- 



taeese and Solanacei3e, etc. (Fig. 60.) 

 Solanum Dulcamara. 



Secondary Growth is the term given to 

 the process which takes place in the 

 stems of most Dicotyledons, increasing 

 their size and giving rise to a set of new 

 tissues known as Secondary Tissues. 

 These as individuals have already been 

 described in the elements of the xylem 

 and the phloem of the fibro-vascular bun- 

 dle, and it here remains to look at the 

 process in general and to describe a few 

 tissues not before spoken of. 



In Monocotyledons there is as a rule 

 no secondary growth, a few exceptions 

 occurring in the Lily family, (Dracienn, 

 Aloes, Smilax), and, in consequence, the 

 Monocotyledons generally possess long 

 thin stems which increase but little in 

 diameter. In Dicotyledons, on the con- 

 trary, there is a regular growth. New 

 tissues are constantly being formed in 

 annular layers by means of the Cam- 

 bium. The Cambium is a meristematic 

 tissue having its origin in the primary 

 meristem of the plerome cylinder, and 

 by its divisions new elements are added 

 to the phloem and the xylem. The cam- 

 bium cells are elongated and have their 

 walls at right angles. They are some- 

 what brick-shaped, with their longest di- 

 ameter vertical. The cambium cells in- 

 ciease by tangential division, at times 

 building up the sieve elements, again 

 forming the woody elements which in 

 general are formefl more rapidly. 



Fiff 63.— Cross section of 6>i/ jrland from leaf 

 of Mentha piperita, b. base, st, supporting: cell, s 

 secreting cells, c. cuticle, E, epidermis of leaf. 

 (Tschirch). 



F 



Medullary Hays. Between the fibro- 

 vascular bundles of most Dicotyle- 

 dons and Gymnosperms thin strips 

 of parenchymatic cell are found. 

 These are the Medullary Rays^ connect- 

 ing the pith with the epidermis. Two kinds 

 are to be distinguished:— Primary medul- 

 lary rays which represent the early di- 

 visions between the bundles, and Secon- 



