48 



'The question still remains to be setlled whether there 

 IS foundation and cause for giving a biological signifi- 



- Purpc 



o f the organism to definite external agents. 



In my opinion, there is no reason v/hatever for this. 

 If we, first of all, confine ourselves to the "shade leaves" 

 it is evident that their mesophyll retains, in more than 

 one respect, the fehaxacter of young, undeyeloped leaves, 

 while in the sun leaves, even these evidences are sooner 

 or later lost by characteristic processes of growth and 

 differentiation. According to Stahl, the mesophyll of the 

 sun leaves of Lactuca Scariola which are oriented vertical- 

 ly, consists throughout of palisade cells j horizontal 

 leaves which get the light only on the side, develop palis- 

 ade ^elis only on the side exposed to the light, while 

 they are entirely absent in leaves grown in shady places. 

 Also some species of Iris do not develop palisade cells 

 when growing in the shade. In other cases, even in shadey 

 places, palisade cells are produced, but in a lesser devel- 

 opment than in leaves exposed to the light. Because the 

 characteristic palisade cells, oriented with their long 

 axis perpendicular to the surface of the organ^ are not pro- 

 duced, leaves grown in the shade remain similar to young, 

 undifferentiated ones, so far as the form of their cells 

 (50) is concerned,! and besides the cells of leaves grown in the 

 shade remind one also, in number and size, of the conditions 

 found in undeveloped leaves, I think that the appearance 

 of such arrested developments in no way proves the ability 

 of the plants and leaves, "to adjust the formation of their 

 assimilatory parenchyma, in a self-regulating manner, to the 

 given intensity of light". ^ For this is needed primarily 

 the experimental proof that a leaf-blade, with mesophyll 

 •free from palisade cells, is capable of a more energetic 

 assimilatory activity in the shade, than is a leaf provided 



1, In many cases the spongy parenchsnaa cells of shade 

 leaves show strong growth parallel to the surface of the 

 leaf. As Stahl also has already indicated (loc. cit» p. 24) 

 this stretching is to be traced back to the action of mech- 

 anical factors, and explained by the energetic grovrth of 

 the leaf ribs, which stretch a little the tissue fields 

 lying between bhe,, The same factors will help to explain 

 also the porosity of the tissue in shade leaves. 



2. Haberlandt, Physiol. Pflanzenanat. , 8. Aufl, , 

 Leipzig, 1896, p, 253, 



