THE STEM. 31 



tions, and is practically impervious to moisture, though not to 

 gases. Protoplasm and cell -sap occupy the interior of the epi- 

 dermic cells, and chlorophyll granules are also found there. Most 

 cells of the kind do not contain such granules. The hairs appear 

 as large multicellular structures growing from the epidermis, and 

 small openings, known as stomcda are present. These are more 

 abundant in leaves, and will he spoken of in connection with those 

 members (p. 65). They are never found in roots. The ground- 

 tissue is mainly composed, as in the root, of parenchyma (cf. 

 p. 14), with cellulose walls and protoplasmic contents; but the 

 parenchyma of the cortex differs from that of the root in the pos- 

 session of numerous chlorophyll granules. There is, besides, in 

 the cortex another kind of tissue, collenchyma. forming a band 

 beneath the epidermis. This differs from parenchyma mainly in 

 the fact that the cells are thickened at their angles. Numerous 

 chlorophyll granules are imbedded in their protoplasm. It is 

 best to examine the vascular bundles in sections of a very young 

 stem, as they are then quite distinct from one another. 1 In a 

 properly stained cross- section (cf Appendix) we shall be able, 

 with a low power of the microscope, to distinguish the bundles as 

 oval areas, each of which is composed of an outer part, the bast 

 or phloem, and an inner part, the icood or xylem, separated by a 

 band of very thin-walled cells known as the cambium. As seen 

 with a high power (fig. 6), the bast is made up of an outer some- 

 what crescentic portion, the hard bast, the elements of which. 

 bast fibres, have very thick walls and no protoplasm, and an inner 

 part, the soft bast, the components of which have thin cellulose 

 walls. Some of these, the sieve tubes, are large, others, making 

 up the bast parenchyma, are much smaller. A number of the 

 sieve tubes present an appearance reminding one of the perforated 

 top of a pepper-castor. This appearance is due to the presence 

 of sieve plates pierced with small holes. The most conspicuous 

 components of its wood are the icood vessels, large rounded ele- 

 ments with very thick walls and no protoplasm. The remaining 

 parts are icood parenchyma and ico.od fibres, the latter closely 

 resembling the bast fibres. The cells of the cambium have very 

 thin cellulose walls and abundant deeply- stained protoplasm. 

 Their shape is rectangular, with the long diameter tangential. 

 and they are arranged in radial rows, two or three cells in a row. 

 These features show that division has taken place in a tangential 

 direction. Imagine a cell, square in cross- section, with two sides 

 at right angles to a radius of the stem. Then suppose this cell 

 to be bisected by formation of a new wall parallel to the two in 



1 In buttercup and some other plants the bundles remain distinct through- 

 out life. 



