GREENFIELD: HISTOLOGY OF THE RUSSIAN THISTLE. 265 



section. These ribs in a stem five millimeters in diameter are 

 0.216 millimeters thick radially, and 0.29 millimeters thick tan- 

 gentially. The lumen of the average collenchyma cell averages 

 0.0175 millimeters radially and tangentially, and 0.65 to 0.34 

 millimeters longitudinally. The v^'alls average 0.0044 milli- 

 meters thick. The epidermal cells above these collenchyma 

 cells are elongated, and there are no stomata here. (Fig. 13.) It 

 is on these ridges that the trichomes appear. They consist of 

 clothing hairs, some 0.2 millimeters long, others so short that 

 they are nearly knobs. All of them have small basal cells. In 

 some cases the basal cells are very .small. (Fig. 11.) In all 

 cases the cuticle of the trichomes is rough. Since the tri- 

 chomes are on the ribs, they overhang the channels to some 

 degree and quite effectively protect that portion of the stem 

 which has the stomata. 



Between the ribs the epidermal cells are more or less isodia- 

 metric, and the stomata are frequent. Their guard cells are 

 arranged transversely to the longitudinal axis. The guard 

 cells have small projections of the cuticle that lap together 

 when the stoma is closed. The lumens of the epidermal cells 

 average 0.07 millimeters tangentially and 0.0175 millimeters 

 radially. The upper wall averages 0.0175 millimeters thick, 

 0.0058 millimeters of this being cuticle. The lower wall aver- 

 ages 0.0029 millimeters thick. 



Beneath the epidermal cells in the channels are palisade 

 parenchyma cells averaging 0.017 millimeters tangentially and 

 0.0525 millimeters radially. Their walls average 0.0014 mil- 

 limeters thick. Beneath these palisade parenchyma cells there 

 is a second row of chloroplast-bearing cells with the plastids 

 massed on the inner side of the cells. These cells average 0.05 

 millimeters tangentially and 0.0175 millimeters radially and 

 longitudinally. The upper walls average 0.0026 millimeters 

 thick and the lower 0.00286^millimeters thick. Figure 5 shows 

 these cells as they occur in the stem. In figures 27 and 28 the 

 corresponding and very similar cells in the leaf are shown in 

 detail. 



Beneath the cells just described and the subepidermal col- 

 lenchyma rib is a strongly developed aqueous tissue, consist- 

 ing of parechyma cells averaging 0.0875 millimeters radially, 

 0.14 millimeters tangentially, and 0.13 longitudinally. The 

 walls average 0.0058 millimeters thick. The tissue is two or 



