156 



BOTANY. 



generally loosely arranged, with large intercellular spaces be- 

 tween them (Figs. 131 and 133), and these are in free com- 

 munication with the external air by means of the stomata. 

 It most frequently happens that this loose tissue is in the 

 under part of the leaf, while the 

 upper portion is composed of one or 

 more layers of closely placed cells ; 

 and this agrees with the general 

 distribution of the stomata, there 

 being usually many more on the 

 under than the upper surface. 



203. — The upper denser tissue, 

 termed palisade tissue, is composed 

 of elongated cells, which stand at 

 right angles to the surface of the 



Fig. 13J.-Sectionof tiie"pali- , » /tti- ioi\ t i.- j.i 



sade "tissue of the leaf of «i- leaf (ilg. 131). in CrOSS-SCCtlOn the 



thTiiVeS^faiJ^^rfe^w ofihe palisade-cclls are cylindrical, with 

 rh"ororh™g4n*et Tmi Small intercellular spaces between 

 Fromadrawmgbyj.c.Arthur. them (Fig. 133), or in some cases 

 they are more or less compressed and angular. 



In general, palisade tissue is confined to the upper surface 

 of the leaf, the lower being occu- 

 pied by the loose tissue previously 

 mentioned ; but there are some cu- 

 rious exceptions to this rule. The 

 most notable of these is found in 

 the leaf of Silpliium laciniatum — 

 the so-called Compass Plant* — of 

 the Mississippi Valley ; its chloro- 

 phyll-bearing parenchyma is almost 

 entirely arranged as palisade tissue, 

 so that the upper and lower por- 

 tions are almost exactly identical 

 in structure (Fig. 134). The ver- 

 tical leaves of the Manzanita of 

 the Pacific Coast {Arctostaphylos pungens, var. platypJiylla) 

 have a similar structure. 



Fig. 138.-Section of the loose 

 parenchyma of the leaf of Echino- 

 cystis lobaia, taken parallel to the 

 leaf surface. Several of the cells 

 are drawn showing their chloro- 



Shyll granules. X 250.— From a 

 rawing by J. C. Arthur. 



* For descriptions of this curious plant, whose leaves have a marked 

 tendency to stand with one edge to the north and the other to the 



