CHAPTER II 



STEMS 



When studying stems it should first be determined whether 

 they were derived from monocotyledonous or dicotyledonous 

 plants. This fact is ascertained by determining the type of 

 the fibro- vascular bundle. See Chapter XI. The next fact 

 to determine is whether the stem is from an herb or from a woody 

 plant. This fact is readily determined because herbaceous 

 stems have a true epidermis, masses of collenchyma at the 

 angles of the stem. The cortical cells contain chlorophyll, and 

 the pith is very large. Woody stems have a corky layer, a 

 phellogen layer, and the pith is very small except in the very 

 young woody stems. 



Having determined these facts, a study should be made of 

 the arrangement, form, structure, color, and the cell contents 

 of the different cells in order to determine the species of plant 

 from which the stem was obtained. 



HERBACEOUS STEMS 



The great variation in the structure of herbaceous stems is 

 shown in the cross-sections of spigelia (Plate 95); in ruellia 

 (Plate 96); in the charts of powdered genuine horehound, 

 powdered spurious horehound, and in the chart of powdered 

 insect flower stems. 



CROSS-SECTION SPIGELIA STEM 



Spigelia stem (Plate 95) has the following characteristic 

 structure : 



Epidermis. The epidermal cells are papillate. 



Cortex. The cortical parenchyma cells consist of tan- 

 gen tially elongated cells which are oval in outline. 



Phloem. The phloem consists of sieve cells, phloem paren- 

 chyma, and c^f bast fibres. 



233 



