THE HISTOLOGY OF THE EPHEDRE^E. 205 



centres. It was found to be possible to obtain photographs both of the light discs and 

 of the images in them of objects held before the mirror (Plate L, fig. 6). The occur- 

 rence of these organs in stems which function to a great extent as leaves is almost to be 

 expected, and the advantage of the erect light position to the desert forms of the 

 Ephedrese is obvious from an oecological point of view. Any movement of the stem 

 from the normal light position will be followed by a transference of the lighted area to 

 another portion of the cytoplasm of the back wall of the epidermal cell. This trans- 

 ference, accompanied as it must be by a variation in the intensity of the illumination, 

 will act as a stimulus resulting in growth movements, whereby the former light position 

 will be returned to. 



Stomata in single or double rows occur in the furrows between the ridges. Each 

 stoma is sunk in a compartment below the surface of the epidermis (Plate J., fig. 7). 

 The entrance to the ante-chamber is more or less constricted by the encroachment of 

 the walls of the four surrounding cells (Plate I., fig. 8). The encroaching wall contains 

 a central core of mucilage, and in E. viridis, where the material had been treated with 

 water, the mucilage had swollen, causing the opposite walls of the chamber to meet. 

 This suggests an adaptation for either narrowing or closing the chamber, and thus 

 limiting transpiration. This is the probable explanation of the closing of the stomatal 

 chambers by resinous masses, referred to by Volckens, and instanced by Stapf (3). 

 The surface shape of the aperture depends upon the degree of encroachment of the 

 projecting walls of the ante-chamber. The guard cells are without the usual ridges of 

 entrance and of exit, as described by De Bary * (4). 



The Cortex is well developed, consisting of chlorenchyma and stereom. The 

 chlorenchyma is differentiated into an outer palisade cortex of radially elongated cells, 

 and an inner spongy cortex of polygonal cells, both having a well-developed aerating 

 system, especially the latter (Plate I., fig. 9). Starch grains occur all through the 

 cortex along with crystals of calcium oxalate, the latter being more abundant in the 

 endocortex, where crystal sacs may occur. Calcium oxalate crystals also occur in the 

 cell walls. In some species a very large proportion of the cortical cells contain tannin, 

 either in the form of mucilage or in the form of small globular masses (Plate I., fig. 2). 

 A sinuous layer of tangentially elongated, closely packed cells, containing large starch 

 grains surrounding the stele, constitutes an endodermis. The innermost layer of the 

 cortex abuts on to this layer at right angles. 



In the stem occur typically four series of strengthening fibres. The walls are at 

 first of condensed cellulose, colouring blue only after prolonged treatment with iodine 

 and sulphuric acid ; afterwards the walls become lignified, and are coloured red by 

 phloroglucin and hydrochloric acid. Excluding the perimedullary stereom system, 

 reference to which will be made later, the remaining three series of fibres are arranged 

 on a girder principle, that is, a broad flange at either end, the hypodermal and 



* The outer and inner walls of the guard cells are lignified from an early date, a condition found in many of the 

 Coniferae. 



