COVILLEA TRIDENTATA. 



The following' measurements were made: 



13 



EPHEDUA ANTISYPHILITICA. (Fig. 6.) 



Rphedra occurs in the wash at the foot of Tumamoc Hill, to the west of 

 the Laboratory. The specimen selected for observation forms a dense shrub 

 about 2 m. high, which has found refuse from predatory cattle by growing 

 under a large Acacia greggii. As is well known, the plant has an appear- 

 ance much like that of scouring rush, which is due to the numerous slender 

 branches that are divided into sections of about 50 cm. each. These 

 branches are the only green ones on the plant; the older ones are covered 

 with a rough bark, which is of gray color. 



The general structure of one of the green branches may be outlined as 

 follows: An epidermis with heavy cuticle and with deeply sunken stomata 

 bound the stem. The stomata are regularly disposed in a manner depend- 

 ing on the arrangement of certain mechanical tissues within the cortex. 

 I refer to bundles of fibers which occur at intervals of about 50 p- on the inner 

 edge and abutting on the epidermis . Between the bundles the surface of the 

 stem is somewhat depressed and in these channels the stomata are placed. 

 The cortex is composed mainly of palisade cells which are chlorophyllaceous, 

 but fibers in groups are scattered in an irregular fashion through the cortex. 

 The wood and the pith in young stems do not contain chlorophyll; in older 

 stems, however, the medullary rays of the wood are supplied with chloro- 

 phyll. 



The younger portions of the green branches, with a diameter of 1 mm., 

 have chlorophyll in the cortex only and, as mentioned above, this is pali- 

 sade. The cells range in length from 15 ^ to 65 /*, and of these the shorter 

 are uniformly near the woody cylinder. In stems 1.5 mm. in diameter the 

 inner cells have lost their palisade character and are more or less cuboid. 

 This is probably owing to the growth in diameter of the stem and to the 

 consequent tangential stretching and radial compression of the cortex. 

 Finally, these inner cells become elongated in a direction parallel to the 

 surface, so that their primitive character is wholly lost. 



In stems 2 mm. in diameter the diameter of the woody cylinder and the 

 thickness of the cortex are noticeably increased as a result of the activity 

 of the cambium. The topography of the chlorophyll apparatus is likewise 



