160 MINUTE STRUCTURE OF THE LEAF. 



chyma ; but the aggregate thickness of these may not be so 

 great as that of the spongy parenchyma on the other side of 

 the leaf (see 205). 



448. In some plants the palisade parenchyma is found almost 

 as abundantly in the under as in the upper portions of the 

 leaves. Bessey * has shown that this is the case in the leaf of the 

 Compass plant (Silphium laciniatum) : "Its chlorophyll-bearing 

 parenchyma is almost entirety arranged as palisade tissue, so 

 that the upper and lower portions are almost exactly identical 

 in structure." Another plant possessing substantially the same 

 leaf-structure is Lactuca Scariola. When its leaves are grown in 

 the light, they take a vertical position (and generally stand north 

 and south) ; but if grown in the shade, they are horizontal. 

 The leaves which are developed in the light have palisade paren- 

 chyma on both the upper and under portions ; 2 but those which 

 are developed in the shade have ordinary parenchyma above 

 and more or less stellate parenchyma below. 



449. According to Stahl, 8 exposure of a leaf to light or shade 

 during development has very much to do in the plants thus 

 far examined with the form and arrangement of its paren- 

 chyma. The leaves of the common beech afford good material 

 for the study of the subject. In some cases, at least, those 

 which are grown in the deep shade of a grove are different in 

 texture from those which are formed in bright sunlight. 



450. The parenchyma of the petiole is generally much like 

 that of the stem to which it is attached ; layers or lines of thin- 

 walled collenchyma sometimes extending without interruption 

 from the stem into the petiole. In the petioles of C3'cads scle- 

 rotic elements like those of the stem are often abundant, and are 

 continuous with them. 4 



451. In some leaves which have the power of movement the 

 petiole is much enlarged at its base, forming what is known as 

 the pulvinus. The parenchyma of this structure is sometimes 

 peculiar in being thick-walled on the upper side of the petiole 

 and thin-walled on the under. Other peculiarities will be de- 

 scribed under " Movements." 



1 See also American Naturalist, 1877. 



2 Pick : Botanisches Centralblatt, 1882, vol. xi. p. 441. 



8 Stahl : Ueber den Einfluss des sonnigen oder schattigen Standortes auf 

 die Ausbildung der Laubblatter, Jena, 1883. 



Haberlandt, on the other hand, does not think the effect of light in con- 

 trolling the character of leaf-structure is well marked. 



* Kraus : Pringsheim's Jahrb., 1865, vol. iv. p. 305. 



