MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 13 



However, the cylinders and their elements vary in the degree of development, and the 

 latter in their general and particular arrangement. 



Taking up their structure and relation to each other, and beginning at the peri- 

 phery, we have: 



1. The EPIDERMIS. A cuticle, whose thickness depends on the age and environ- 

 ment of the individual, covers the exposed surface of the epidermal cells in all cases. 

 The epidermis is either simple or double ; in the latter case there are two layers of quite 

 similar cells. A slight modification occurs in section Avicularia, Avhich will be men- 

 tioned under that head. 



As a rule, the two usual modifications of epidermal cells are present. These are 

 stomata and trichomes. The former were not plentiful in any of the examples studied, 

 but a greater or less number are developed on the younger parts. As regards the latter, 

 we find both non-glandular and glandular trichomes ; the former, however, predominate, 

 and as with the stomata, the trichomes are characteristic of the younger parts of the 

 epidermis, although in a few cases they do exist even on the oldest portions. In some 

 cases the non-glandular trichomes appear as conical unicellular hairs, sometimes de- 

 pressed, sometimes elongated, but always Avith a blunt apex. Then there are long 

 bristle-hke hairs, also of one cell, but with a long attenuated apex. In other cases we 

 find trichomes similar to the last in all respects save their being multicellular. As a 

 final modification, in two sections we find simple or branched hairs. They are multi- 

 cellular, and fork either at the base or above it into two or several prongs. 



The glandular trichomes consist of black or yellow glands situated at the ends of 

 unicellular hairs. 



The epidermis is the first element of the cellular tissues. To anticipate a httle, it 

 may be well to mention regions of these tissues as well as the remaining elements. Be- 

 sides the epidermis, cellular tissue is represented in the primary cortex by the elements 

 of the hypoderma, the large parenchymatous cells of the starch-ring, the accompanying 

 tissue of the sieve-tubes, and finally the most typical and abundant development takes 

 place as pith. The other elements and tissues always present are as follows : Next to the 

 starch-ring there is a greater or less development of sclerenchymatous cells which in most 

 cases form a stereome cylinder; numerous sieve-tubes exist in the phloem regions, and 

 within these is the xylem, made up of its different kinds of tracheae, woody parenchyma 

 and intermediate tissue, all combined into typical collateral bundles. 



2. The periblematic cylinder. This zone of the stem contains at least two main 

 tissues in every instance, and in two cases a third one appears. Following the epidermis 

 is the primary cortex consisting of a hypoderma of one or more rows of cells which passes 



