2o6 



BOTANY AND PHARMACOGNOSY. 



On comparing the primary structure of the root with that of 

 the stem, the following essential differences are observed : 



Epidermis with root hairs. 



Cortical parenchyma without chloro- 



plastids. 

 Fibrovascular bundles, radial, more 



numerous in monocotyledons than 



in dicotyledons. 

 Pith usually wanting. 



Epidermis. 



Cortical parenchyma with chloroplas- 

 tids. 



Fibrovascular bundles, open collateral 

 (in Dicotyledons), or closed col- 

 lateral (in Monocotyledons). 



Pith always present. 



Secondary Structure. — The development of the secondary 

 structure in dicotyledonous stems is quite simple compared to 

 that of the root. The cambium develops into a continuous ring, 

 producing xylem within and phloem without. A phellogen arises 

 beneath the endodermis, and the cork replaces it and the over- 

 lying primary tissues (Figs. 115, 116). 



The arrangement of the tissues in the primary and secondary 

 structures of dicotyledonous stems may be compared as follows : 



Primary structure: Epidermis. Hypodermis. Primary 

 cortex. Endodermis. Fibrovascular bundles, open collateral 

 and few. Pith. 



Secondary structure: Cork. Phellogen. Secondary cor- 

 tex. Fibrovascular bundles, open collateral, numerous and 

 separated by secondary medullary rays. Pith cells frequently 

 disintegrated. 



Monocotyledonous stems, like monocotyledonous roots, 

 usually grow in length, without much increase in thickness, ex- 

 cept in certain cases, as in rhizomes and palm stems, and retain 

 for the most part the primary structure (Fig. 114). The fibro- 

 vascular bundles are distinct and distributed in the form of small 

 circular groups throughout the parenchyma within the endo- 

 dermis as well as in the cortex. The bundles are usually of 

 the collateral type and spoken of as closed from the fact that there 

 is no cambium between the xylem and phloem. In some cases the 

 phloem is in the center of the bundle, the xylem forming a con- 

 ceritric ring around it. In some monocotyledons these two types 

 of bundles are associated (Fig. 114), those of the concentric type 



