STEMS, ROOTS, AND LEAVES 105 



HERBACEOUS STEMS 

 DICOTYLEDONS 



In the following account of the structure of herbaceous dicoty- 

 ledons the stem of a common cultivated sage {Salvia) will be 

 taken to illustrate the usual structure of one type of herba- 

 ceous stems. In studying sections through portions of young 

 stems of Salvia (Fig. 56, a) and through the older basal portions 

 (Fig. 56, 5) the student will be impressed with certain broad 

 distinctions between such a type of herbaceous stem and the 

 woody stems already considered. These general distinctions are 

 necessarily concerned with the main tissue areas common to all 

 stems ; namely, the bark, cortex, and pith, the cambium, and 

 the vascular cylinder. 



The bark is evidently quite lacking in Salvia, if by this term 

 we refer only to the thick jacket of bark which clothes the out- 

 side of woody-stemmed plants. The epidermis is therefore per- 

 sistent throughout the season and serves its usual functions of 

 checking the loss of water from the delicate tissues beneath it 

 and of protecting the stem from insects and fungi. 



The cortex of the Salvia stem is a wide layer, as in most 

 strictly herbaceous stems, and in the younger portions its cells 

 contain green chloroplasts which function in photosynthesis. The 

 cortex cells are also differentiated into a narrow outer thick- 

 walled strengthening cylinder and a wider inner cylinder made 

 up of more delicate cells, which perform the work of photosyn- 

 thesis and storage. The pith is relatively larger than in woody 

 stems and serves the usual storage function. 



The vascular cylinder is perhaps the most distinctive feature 

 of the stem of Salvia as compared with the thick vascular cylinder 

 of trees and shrubs. In sections of young stems (Fig. 56, a) the 

 vascular tissue, composed of phloem and xylem, is seen to occur 

 in isolated strands, called vascular bundles, united by thin bands 

 of fiberlike cells, easily distinguished from the adjacent cells of 

 the pith and cortex by their smaller size in transverse section 

 and by their considerable length as seen in long sections. These 

 connecting bands or plates of fibrous tissue, together with the 



