GREENFIELD: HISTOLOGY OF THE RUSSIAN THISTLE. 267 



ters radially and tangentially, and is from 0.105 to 0.21 milli- 

 meters longitudinally. The average tracheae have walls 

 0.00437 millimeters thick. The lumen is 0.0437 millimeters 

 radially. The tracheids are often irregular in shape. (Fig. 

 21.) Their walls are about the thickness of the walls of the 

 tracheae, and their average length is 0.13 millimeters. 



Medullary rays are not found in the old stem of Salsola kail 

 var. tenuifoUa, nor, according to Solereder (1908), are they 

 found in the stem of any species of the family. In families 

 which have similar anomalies medullary rays are found in a 

 number of species. 



The pith is composed of exceedingly large, pitted cells 

 avei'aging 0.216 millimeters in diameter. The walls are 0.0086 

 millimeters thick. In a great many of the cells are calcium 

 oxalate crystals, not clustered as those in the aqueous tissue, 

 but scattered in the cells. 



In a stem 1.36 millimeters in diameter 2.8 per cent of the 

 tissue is devoted to water transportation. 



THE LEAF. 



The leaf of Salsola kali var. tenulfolia is narrow, terete and 

 succulent. The epidermis consists of a single layer of cells. 

 The lumens of these cells average 0.0175 millimeters radially 

 and 0.065 millimeters tangentially. The outer wall is 0.0058 

 millimeters thick, 0.0019 millimeters of this being cuticle. The 

 inner wall is 0.00284 millimeters thick. The stomatal open- 

 ings are arranged parallel with each other and transversely 

 to the median vein of the leaf. The hairy covering is very 

 similar to the hairy covering of the stem. The trichomes near 

 the base are longer than those near the tip of the leaf. Uni- 

 cellular trichomes are not found, but uniseriate trichomes are 

 frequent. 



Immediately beneath the epidermis is a layer of palisade 

 parenchyma cells. (Figs. 25, 30, and 31.) Figure 27 shows 

 these cells in detail. The lumens of these cells average 0.0525 

 milhmeters radially and 0.0172 millimeters tangentially. The 

 walls average 0.00144 millimeters thick. Below the palisade 

 parenchyma cells is a row of short chloroplast-bearing cells 

 with their chloroplasts massed toward the inner side of the 

 cells. (Figs. 27 and 28.) The lumens of the cells average 

 0.0262 millimeters radially, 0.035 millimeters tangentially, and 

 0.0262 millimeters longitudinally. The upper wall averages 



