MEMOIRS FROM THE DEPAETMENT OF BOTANY OF COLUMBIA COLLEGE. 171 



I 



A simple epidermis encloses the primary cortex. The cells are parallelopipeds hav- 

 ing a diameter of from 10-20 fj. and a height ranging from 40-70 (x. In cross section 

 they vary on either side of equilateral, the long axis sometimes being tangential, some- 

 times radial. Just within the epidermis, at intervals corresponding radially with the 

 collateral bundles, we find fascicles of sclerenchymatous cells, which are bounded within 

 by either the ordinary cells of the primary cortex or by a layer of epidermal-like cells. 

 These fascicles, as will be seen from the plates, both in their position and structure are 

 exceptionally fine illustrations of the mechanical arrangement of the stereome, and pro- 

 duce the ridges so conspicuous on many of the stems in subgenus Avicularia. Following 

 the epidermis and the above-mentioned sclerotic elements is the primary cortex, which 

 varies in thickness from two to eight cells, the arrangement of which differs from all other 

 specimens examined; instead of having their long axes parallel to the length of the 

 stem, they lie in the opposite position and contrary to what we usually see ; in cross sec- 

 tion the cells appear oblong or vermiform, while in long section they are more or less 

 polyhedral and isodiametric ; their width is a little less than one-half that of the epi- 

 dermal cells. The starch-ring, whose continuity is occasionally interrupted by the 

 bundles of stereids, is made up of prismatic or lenticular cells ranging from one to three 

 layers deep. The cells of the outer layer, which are larger than the rest, are somewhat 

 shorter and a little more irregular than the epidermal cells. 



The stereome does not form a cylinder in this subgenus, as it does in most others, 

 but consists of fascicles ranging from 50-100 fi in diameter and separated by the elements 

 of the starch-ring. These fascicles and those in the periblematic cylinder are of about 

 the same size and correspond radially. Their stereids are of the normal type in the genus, 

 varying from 10-20 [x in diameter, with the cavity almost obliterated and ranging in 

 length from 170-260 (i. Just within the bundles of the stereome there occurs a great 

 development of phloem; the elements are remarkably small and seldom reach 10 ^t^ in 

 diameter. The phloem is thickest about the bundles of sclerotic cells, whence it gradu- 

 ally thins out but forms an almost continuous sheath. In long section the sieve-tubes 

 are distinct and measure from 150-200 fx in length, and the septa are usually somewhat 

 oblique. A strong growth of xylem is opposed to this sheath; the woody fibres are 

 typical, mostly having tapering ends and varying from 200-250 fi in length ; in cross 

 section they are either square or oblong ; in the latter case the long axes are radial ; their 

 diameter ranges from 5-15 ^. Scattered throughout are the larger elements, which 

 appear more or less prismatic in cross section, measuring from 15-40 ^ in diameter. 

 Towards the outside of the bundles we find narrow tracheides with small border pits, 

 and a little further in compact spiral vessels. As we approach the region of the wood- 



