THE STEM OF THE BASS WOOD OR LINDEN 111 



From what has already been said of the structural characteristics 

 of the tracheary cells of the xylem, it is almost unnecessary to 

 state that the limits between secondary xylem and metaxylem are 

 virtually impossible to determine when considering only trans- 

 verse sections of a vascular bundle. In the interval between each 

 of the individual bundles in the ring, there occurs a zone of small 

 thin-walled, obviously meristematic cells which are continuous 

 with the cambial zone in the bundles themselves and represent 

 what is termed the interfascicular cambium. The interfascicular 

 cambium originates by the tangential division of cells in the 

 medullary rays adjacent to the strips of fascicular cambium. In 

 certain plants, the interfascicular cambium only forms paren- 

 chyma (e.g., Clcmatic, Aristolochia) but in Pelargonium, as in 

 many herbaceous plants, the interfascicular cambium gives rise 

 to additional "bundles" composed entirely of secondary phloem 

 and secondary xylem ; the extensive development of vascular 

 tissue from the interfascicular cambium usually results in the 

 formation of a complete cylinder of secondary xylem and phloem. 

 In studying the sections note that phloem is the first vascular 

 tissue to differentiate from the interfascicular cambium and is 

 later followed by the centripetal formation of secondary xylem ; 

 whether this is an exceptional condition can only be determined 

 by the investigation of a large number of herbaceous stems. 



The center of the stele is occupied by the pith, which is com- 

 posed exclusively of large isodiametric thin-walled parenchyma, 

 cells separated from one another by conspicuous intercellular air- 

 spaces. The extreme abundance of starch grains in the cells of 

 the pith indicates that this region has as its function the storing 

 of reserve food material. 



2. The stem of the hasswood or linden (Tilia sp.). Obtain a 

 stained slide with transverse, radial and tangential sections of the 

 stem. Examine first the trans-section and study the following 

 tissues and regions from the periphery of the stem to its center, 

 viz. : 



(a) The epidermis, a uniseriate layer of cells which appears 

 broken and cracked in numerous places due to the development 

 of a prominent periderm beneath it. Structurally the cells of the 

 epidermis are rather small, are oval in shape (in section view) 



