50 



cLevelof)ments . i. e. tissues scantily deyeloped as compared 

 with the "nortnal " ones. 



Fig. 12, Showing a cross-section through the leaf tip 

 of the land form of Ranunculus fluitans side by side with 

 that through; the leaf of the water form, should make pos- 

 sible a comparison between the leaf structure of plants 

 knovm as typical v;ater dwellers, and such as become "water 

 plants" only thruough the compulsion of the experiment, or 

 of unfavorable external conditions. In both cases mesophyll 

 cells of very simple round form are produced. This corres- 

 pondence however, makes so much the lesss superfluous the 

 experimental proSf that the round cell functions better under 

 such air conditions as are offered to submerged parts of 

 plants, than do the palisade cells, since the appearance of 

 that simple cell form shows, as we have seen, no specific 

 ' effect of the diffuse light and the life under water. What 

 complicated accessory suppositions v/ould become necessary 

 for the preparation of teleogical explanations, if those 

 mesophyibl structures resembling shade leaves should now be 

 considered also in the light of appropriate reactions, pro- 

 duced under the influence of the too great drought, under that 

 of a lack of carbon dioxide, tipon the action of animal para- 

 sites or upon other disturbances in their nutrition? 



In my opinion similar eonsiderations stand in the way 

 also of the biological explanation of other arrested devel- 

 opments . 



The stems of Cardamine growing under vmter develop, 

 according to Schenck, no mechanical tissues; the "formation 

 of these is unnecessary under water, for the water itself by 

 means of its greater density keeps the plaat in position 

 favorable for light. "^ How is it, however, with those 

 plants which mature in moist air and do not develop mechan- 

 ical tissue, or indeed, with those experimental plants of 

 Thouvenin's, although for them it would have been just as 

 "necessary " or indeed, more than necessary than for the 

 specimens living under normal conditions? Further t through 

 . a scanty development of the medullary ray, the vascular 

 bundles in the water form of Cardamine move somewhat toward 

 /t:o\ the center, "a tendency which in tj^pical water plants J^as 

 ^ ' led to the formation of axillary vascular fibres." According 

 to Schenck s'^oh an arrangement is "purposeful" for water 

 plants, Since by this means the tensile strength of the ax- 

 illary parts developing- under water is increased. It may 

 seem here as if the variation of the water form from the 

 normal should he explained as a purposeful transformation. 

 From my point of view, however, this cannot enter into the dia 

 cussion. because keeping the plants in standing water cannot 

 be of ejual significance with keeping it in running water Mr 

 only in the latter is the tension produced. Besides, the 

 reduction of the pith, by which the vascular bundles seem 

 shoved out of place towards the center, takes place also under 

 other cultural conditions; for example, in strongly etiolated 



plants. _ _ -,- 



1, Ueber Strtiktur&nderung, u. s. v;, Loc. Git. p. 483, 



