THE HISTOLOGY OF THE EPHEDRE^E. 209 



fig. 22). Certain of the pith cells in the neighbourhood of the protoxylems show 

 greenish-coloured contents, which seem to be chlorophyll. The cells of the pith have 

 pitting on all their walls, especially on the end walls (Plate III., fig. 23). 



Meristematic Tissues. — The stem apex, enclosed in several whorls of leaves, consists 

 of a small conically-shaped mass of tissue. According to Dingler (8), growth is from a 

 tetrahedral apical cell. In all the material examined the meristem has been found to 

 be stratified. Different records in regard to the nature of the apex have, however, been 

 made, and De Bary (9) mentions that both forms of apex occur in the same species. 

 In the first internode after the apical the stelar elements have begun to differentiate, 

 while in the second internode they are well developed. In this internode elongation 

 of the stem has set in, but is more pronounced in the succeeding two or three inter- 

 nodes. At first the whole internode is meristematic (Plate III., fig. 17), but later 

 elongation comes from a meristem situated at the base of the internode. This basal 

 meristem remains functional for some considerable time, eventually passing over into 

 permanent tissue, with the exception of a layer of from two to three cells in thickness, 

 situated just above the node. This layer functions as a dehiscence layer (Plate III., 

 fig. 16). 



Part II. — Histology applied to the Determination of Species. 



Considerable attention has recently been paid to the use of the internal structure of 

 the vegetative organs of plants in the diagnosis of orders, genera, and species. The 

 genus Ephedra, consisting as it does of a uniform oecological group of species, seems to 

 furnish a fair test of the value of such diagnostic characters. In his paper on Gnetacese 

 and Coniferae, Bertrand (10) was of opinion that the histological features of stem and 

 leaf were insufficient, and not thoroughly trustworthy for specific distinctions. This 

 opinion is also held by Dr Stapf in his monograph, " Die Gattung der Ephedrege," in 

 which he constantly emphasises the extreme variability of the histology of the vegeta- 

 tive organs. While the tendency to variation in the histology is admitted, it has yet 

 been possible, as a result of the above-mentioned notes, to divide the genus into sub- 

 generic groups. Further, the additional evidence furnished from the internal structure, 

 when taken with the ordinary external morphological characters, greatly facilitates 

 diagnosis. The idea with which this part of the work was undertaken was to work from 

 some definite level, and thereafter to include such other features from other parts of 

 the stem as might seem to be of value. The second elongated internode from an 

 apical bud was taken to start with, as being the youngest in which the complete 

 primary stelar structure is attained. A general division of the group can be made, 

 basing the evidence on the vascular supply of this internode. The number of bundles 

 is very constant in some species (eight in foliata, Helvetica, procera, distachya ; ten 

 in viridis); while in a few there is a slight tendency to vary (nebrodensis and 

 gerardiana), and in even fewer variations actually occur (trifurca). These variations 

 are never of such an extent that they interfere with the value of the classification, for 



