49 



with palisade cells.-'- In any case, it has not yet been shown 

 that shade leaves v;ith their porous mesophyll structure supply 

 the leaves v\?ith a strong transpiratory current. G eneau de Lamar- - 

 liere^ found, on the contrary, that sun«-leaves transpire more 

 strongly than shade leaves under similar external conditions. 

 That sun leaves transpire more strongly in sunshine, that shade 

 leaves in the shade seems fe matter of course and even the scanty 

 supplying of food substances to those leaves grown in shade ex- 

 plains the fact that their tissues do not develop so luxuriantly, 

 nor are they so completely differentiated as the mesophyll of the 

 strongly transpiring and thereby well-nourised leaves, unfold- 

 ing in sunshine. 



If we recognize in the formation of shade leaves, not an 

 adjustment to definite light conditions, but only the unavoid- 

 able product of some arresting factors, the correspondence of 

 shade leaves with leaves of plants from Alpine habitats, as 

 bf ought forward by Leist, loses its remarkableness and v/eneed 

 no complicated explantation for the fact that land plants, 

 placed under water, develop leaf-blades with the homogeneous 

 structure of "shade-leaves". Schenck (loc. cit. p. 484) seems 

 indeed to find in phenomena of the laa*t kind also a purposeful 

 structure adjusted to the abnormal conditions:- "the submerged 

 plants live in a medium, which absorbs the rays of light more 

 strongly than does the air; in a medium which places only ^^^~ 

 fuse light at the disposal of plants living in it. Water plants 

 as well as shade plants must consequently be retarded so^^far as 

 the development of the assimilatory tissue is concerned. ix, 

 however, the action of moist air is enough to produce the same 

 homogeneous tissue structure, if the factors effective in Aipme 

 regions, as yet insufficiently analysed l3]^^„in any case not con- 

 nected ^ith weakened light, ^ 6an- develop ^he same tissue form 

 in certain plants, we will be able to attach very Ixttle vaiue 

 to explanatory experiments of this kind. In my opinion,up to 

 the present, no reason exists for recognizing the mesophyll ^xruo 

 ture of shade leaves, and the parts of plants grown ^^der water 

 as anything other than arrested, (continued on page 50) 



1. Stahl characterized it as "right well conceiyabl'e" (loc. 

 cit. p. 37) that with very weak light, preference wxll ^^^^^hown 

 for shade leaves, since the ability to bring the chloropnyix 

 grains into the favorable position,- the horizontal P°^^J^°^"„j.+ 

 makes possible a more productive utilization of the saanty jlib 

 than can be the case in the tough sun-leaves, whose chloropnyxx 

 grains take up a position less favorable for weak light. 



2. Rech. physiol. s. 1. feuilles devel. a 1' ombre et au 

 soleil. Rev. gen, de Bot., 1896, Y. VIII, P/ 481 



3. The investigations as to the influence of Alpine life 

 conditions on the tissue formation of plants have lea to vex^ 

 dissimilar results in different places and in the testing oi 

 different plants. Compare especially Wagner, A., Zur ^.enntn. 

 des Blattbaues der Alpenfl. u. desen. biolog. Bedeutung. bii^z- 

 ungsber. Akad. Wiss. Wien, 189E, Bd. CI. 



