SECONDARY STRUCTURE. 



135 



SECONDARY STRUCTURE. 



386. It has been noticed that the flbro-vascular bundles of 

 monocotyledons differ from those of dicot3-ledons chiefly in the 

 possession by the latter of a layer of merismatic tissue (cambium) 

 between the cribrose and woody portions. The stems of peren- 

 nial dicotyledons increase in thickness from year to year chiefly 

 by the annual production of a new mass of wood upon the in- 

 side of this layer, and of liber 



upon the outside ; but the stems 

 of most monocotyledons have no 

 provision for annual increase in 

 diameter. Hence it is convenient, 

 in spite of numerous anomalies, 

 to consider the secondary struc- 

 ture of the stem under these two 

 heads. 



387. Secondary structure of 

 moiiocot) ledouous steins. As has 

 been already observed, the pri- 

 mary bundles in palms run from 

 the leaves in curves of long ra- 

 dius until they again approach 

 the surface of the stem, and their 

 fullest development is found in 

 the middle part of their course. 

 While a cross-section exhibits 

 these bundles as scattered without 

 much order in a mass of paren- 

 chyma, a vertical section shows 

 that they have entered the stem 

 at different heights (since the 



leaves with which they were developed were at different points 

 on the stem). A vertical section can display only parts of most 

 of these curved bundles. At the stem of a palm just below the 

 crown of leaves there are as many bundles seen in a cross-sec- 



5th Type. A central mass of secondary tissue, formed from central meris- 

 tem. Intermediate zone, well developed ; e. </., Triglochin maritimum. 



6th Type. Bundles having several distinct liber elements; e. g., Tamus 

 communis. (Anatomie de la tige des Monocotyledones, Ann. des Sc. 

 nat., ser. 6, tome v., 1878, p. 1.) 



FIG. 111. A diamond-shaped mesh of primary fascicles intermingled with secondary 

 fascicles in the stem of an Opuntia. (Keinke. ) 



