53^ 



ECOLOGY 



Fig. 772. — A longitudiaal section 

 through a portion of the stem of 

 .5o/«:orttMi, showing oblique palisades (^), 

 and also a "storage" tracheid (t); the 

 arrow is directed toward the stem apex; 

 jf, stoma; considerably magnified. 



to the leaf surface, regardless of the 

 position of the leaf in relation to light. 

 The position of the palisade layers 

 seems relatively fixed in most cases 

 (not in Lactuca and in Eucalyptus), a 

 rudimentary palisade region often 

 being discernible in the bud; in sub- 

 sequent development the thickness of 

 the layer, but not its position, may 

 vary as external conditions vary. 

 Thus light appears to affect chiefly 

 the cell length, the region of develop- 

 ment and the orientation of the pali- 

 sade cells being due to other and 

 mostly unknown causes. The influence of light upon cell elongation 

 might be conceived of as direct or indirect. In many but not in 

 all albescent leaves the palisades stop sharply at the edge of the 

 green tissue (fig. 773), 

 although the green and 

 colorless spots are ex- 

 posed equally to the 

 light; thus palisade de- 

 velopment appears to 

 be correlated with the 

 formation or with the 

 activity of chlorophyll 

 rather than with light 

 directly. Again, the 

 palisade length in- 

 creases and decreases 

 with the amount of car- 

 bon dioxid, seeming to 

 indicate that palisade 

 size is associated with 

 synthetic activity. 



Palisades and trans- 

 piration. — Not all cases 

 of palisade development 

 can be referred to light, 



Fig. 773. — A cross section of an albescent leaf of 

 Abutilon; to the right of the vein (v) is a chlorophyll- 

 bearing region with two rows of palisade cells (p) and 

 about three layers of sponge tissue (/); to the left is a 

 colorless portion entirely without palisade cells, this part 

 of the leaf having remained in its original undifferentiated 

 state; the lower epidermis (e) contains chlorophyll, even 

 in the albescent region of the leaf; note the lack of 

 symmetry, due to dilferential growth; h, branched epi- 

 dermal hair; s, stoma; v, vascular tract; considerably 

 magrufied. 



