102 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



of sectioning. According to authors quoted by Solereder (84, pp. 259-260), similar sacs 

 in other members of the family may contain protein and other substances in addition to 

 the tannin. Between the tapering lower ends of the tannin sacs are found the true pali- 

 sade cells, which never reach the upper epiderm. Nearly all of the true mesophyll is 

 palisade-like in character, the only differentiation being moderate looseness and irregularity 

 near the lower epiderm. (3) Lower epiderm papillate and upper slightly so. (4) Stomata 

 on lower side only, sunken to the depth of the epiderm, with small exterior chambers. 

 (5) Tannin sacs: see above. 



Rhamnus californica. — (2) Bifacial. Palisade tissue three to four layers deep, occupy- 

 ing slightly more than half the mesophyll; sponge loose. (3) Some cells of upper epiderm 

 doubled, and occasionally one with an apparently gelatinized inner wall, as reported by 

 Herzog (42). (4) Stomata on lower side only, not specialized. (5) Tannin in bundle- 

 sheath; a rather thick covering of clustered hairs on lower surface. 



Rhamnus crocea (fig. 31). — (2) Bifacial. Palisade tissue two to three layers deep, occu- 

 pying half the mesophyll; sponge very loose. (3) Lower epiderm two to three times as 

 thick as the upper, composed of cells elongated perpendicularly to the surface of the leaf, 

 many of them being doubled; appearing like weakly developed water-storage tissue. (4) 

 Stomata on lower side only, barely sunken. (5) Tannin in bundle-sheath. 



33 



34 35 



Fig. 33. — Ceanothus papillosus: diagrammatic section of leaf. X15. 

 Fio. 34. — Ceanothus rigidus: section of leaf, showing a single cavity 



and its surroundings; general features are like those of 



C. cuneatus (fig. 32). X125. 

 Fio. 35. — Ceanothus sorediatus: section of leaf. X125. 



Ceanothus cuneatus (fig. 32).— (1) One of the thickest of the leaves studied. (2) Bi- 

 facial. Mesophyll very complex (Solereder, p. 886; Gemoll, 32). Palisade tissue about six 

 layers deep, not continuous horizontally, occupying the areolae between the closely anasto- 

 mosing veins, the courses of which are prominently marked by strands of tannin cells; 

 dense, but becoming looser and simulating sponge in center of leaf. Mesophyll extends 

 to lower surface around the cavities (see below), in the lower half being of rather dense 

 sponge nature, with a tendency toward palisade-form near the epiderm. (3) Upper epi- 

 derm partially and irregularly double, the inner cells mostly large. 1 Lower epiderm, 



1 Gemoll (32) states that all the cells of the upper epiderm have gelatinized inner walls, and 

 says nothing of doubling. The interpretation given here seems not to be open to doubt, however. 

 My material was killed and preserved in formalin-alcohol and cut by the celloidin method, while 

 Gemoll obtained his from dried herbarium specimens. 



