96 



VEGETABLE HISTOLOGY. 



There is no cambium in the bundles, hence fern stems, just as 

 those of monocotyl plants, do not increase in diameter from 

 year to year. They remain rather slender. 



Note the plan of this stem. It is the type on which all ferns 

 are built. Note also that it differs from the monocotyl and 

 dicotyl types of stem. The two large masses of dark fibrous 

 cells in the central portion of the section are not present in all 

 ferns. 



II. Variety with xylem surrounding phloem. 



Make cross-sections of stems of False Solomon's Seal or 

 Sweet Flag, both monocotyl plants, and stain with phloroglucin. 



The structure of the bundles is just the reverse of that of the 

 bundles of the fern, in that the xylem cells are on the outside and 

 the phloem is central, but 

 there is no endoderrnis pres- 

 ent. In a few bundles the 

 xylem ring is incomplete 

 and the phloem is continu- 

 ous with the parenchyma 

 outside the bundle. The 

 concentric bundles, with 

 phloem central, may be re- 

 garded as closed collateral 

 bundles in which the xylem 

 has completely grown around 

 the phloem mass. 



Both these plants have the 

 arrangement of the parts 

 characteristic of mono- 

 cotyl stems. In both there 

 is a cylinder sheath divid- 

 ing an outer border or cortex from the broad central area in 

 which the bundles are scattered. The parenchyma tissue of 

 Sweet Flag (Calamus) has been studied earlier (Chapter 

 XVIII). The parenchyma of False Solomon's Seal is large, 

 thin-walled, closely packed. Neither section contains cork cells. 



RADIAL BUNDLES. 



These vary considerably among themselves in regard to the 

 number of xylem rays and their length, the amount of lignifica- 

 tion of the cells, the structure of the pericambium layer and of 

 the endodermis. The number of rays varies from two to forty 

 or fifty in different roots. The number of rays is indicated by 

 the word arch with a numeral prefixed. Thus a bundle with 

 two xylem rays is called a diarch bundle, one w T ith three a 

 March bundle, etc. As a rule, dicotyl and gymnosperm roots 

 have fewer rays and a thinner-walled endodermis than roots of 

 monocotyls. 



Fig. 59. Cross-section of a concentric bun- 

 dle of the rhizome of Sweet Flag, with 

 central phloem surrounded by the xylem 

 vessels (Tschirch). 



