144 



PLANT ANATOMY. 



cording to the number of the bundles 

 the roots are known as diarch, triarch, 

 tetrarch, etc. Few of the official roots 

 have radial bundles, Sarsaparilla, Aris- 

 tolochia. (Fig. 56.) 



2, The Concentric Bundle is one in 

 which one of the elements encircles the 

 other more or less completely. In some 

 cases the Xylem is in the centre and 

 surrounded by the phloem, (Aspidium). 

 In others the phloem is in the centre and 

 is surrounded by the Xylem. (Calamus, 

 Iris, etc.) (Fig. 57.) 



growing tissue, the Cambium, which sep- 

 arates it from a zone of combined phloem 



Fig, S9.— Closed Collateral bundle from the leaf 



of Calamus, <7. vessel portion, w. sieve portion, 

 (de Bary). 



Concentric bundles are more common 

 in Monocotyledons, where they form 

 strings of tissue irregularly disposed in 

 the parenchymatic tissues. 



3. The Collateral Bundle is charac- 

 terized by the Xylem and phloem lying 

 side by side in a tangential direction, 

 the xylem being towards the centre. 

 This is the type of bundle of most Pha- 

 nerogamic stems and roots. Three forms 

 of collateral bundle have been described. 



a. The Open Collateral. 



b. The Closed Collateral. 



c. The Bi-collateral. ■ 



a. The Open Collateral bundle is char- 

 acteristic of Dicotyledons. The centre of 

 the root or stem and generally the larger 

 part of it IB occupied by an indefinite 

 number of Xylem elements radiating 

 from the axis, forming a solid cylinder. 

 This is surrounded by a thin layer of 



Fig*. 60. — Cross section of two year old stem of 

 Solanum Dulcamara showing Bi-CoUaterul 

 bundles, is inner sieve portion, /r. outer sieve 

 portion in secondary baric with camblnm and 

 vessel portion between, rs^ mg. medullary rays, 

 in sieve portion and vessel portion, respectiveJy. 

 <Tachirclij . 



