OTHER STEMS — THREE TYPES OF BUNDLES 261 



one year old. Stain with hematoxylin. Make a permanent 

 mount. Study with low power, and make a sketch show- 

 ing the shape and location of the fibro- vascular bundles. 

 Fig. 402. Save the mount for further study. If meni- 

 spermum stems are not easily ob- 

 tained, ivy (Hedera helix) or clem- 

 atis may be substituted. 



423. OTHER STEMS.— Besides the 

 two types of stems studied above, 

 which are prevalent among pheno- 

 gams, there are other structures of 

 stems found among the cryptogams. 

 A common arrangement of the bun- 

 dles is in the form of a circle some 

 distance from the center, with a few 

 other bundles within the circle. 



-^,,.,, • ,1 • , 1 ,. 402. Arrangement of tissues in 



Within the circle also are sometimes 2year-oid stem of moonseed. 

 found large areas of fibrous tissue. l^jf^/J^^^^^i^:. 



Fig. 403. There are, however, wide vascular bundles are very 



prominent. 



variations from this arrangement, 



but this mode of arrangement is often called the fern 



type of stem. 



424. THREE TYPES OF BUNDLES.— It has already been 

 said (418) that every fibro-vascular bundle is made up of 

 two parts: (1) phloem or soft bast; (2) xylem or tvood. 

 The relative position of these two strands of tissue is very 

 important. There a-re three plans of arrangement, on 

 which three types of bundles are based. These plans 

 are collateral, bi-collateral and concentric. 



425. In collateral bundles, the phloem and xylem are 

 placed side by side, the xylem being nearer the center 

 of the stem and the phloem outside or nearer the cir- 

 cumference of the stem. We find this plan in the stems of 

 phenogams. The collateral bundles may be either open or 

 closed. Open bundles are those which continue to increase 



