1902] CHANGE OF FORM IN PROSERPINACA PALUSTRIS 107 



8. Contact stimulus. — The young primordia in the sub- 

 merged plants are intimately surrounded by water, and it is pos- 

 sible that there might be some contact influence here which 

 checks lateral growth and causes^ the elongation of the lobes. 

 Contact stimulus (stereotropism) is well known among the 

 lower animals, as, for example, where the rhizoids of some of 

 the hydroid pol^^ps develop as a response to a stimulus of con- 

 tact from a foreign body. That this cannot be the cause, or any 

 essential part of the cause, is shown by the fact of the formation 

 of the water form in moist air, and also from the facts just men- 

 tioned, where this contact is present but the air form is pro- 

 duced. 



As to the purely anatomical features of the plant, experi- 

 mental work is being carried on. A large mass of facts regard- 

 > ing the appearance of the various anatomical features under the 



diverse conditions of experiment has accumulated and will be 

 presented later. One point only need be mentioned. Besides 

 the positive anatomical characters of the water form, there-are 

 what we may perhaps term negative characters, /. <?., an absence 

 of certain characters that usually accompany the air conditions. 

 These are the absence of such structures as lignified and cuticu- 

 larized tissues, stomata, mechanical tissue, and conducting tissue. 

 Experiments are under way to determine to what extent such 

 tissues are dependent upon definite external stimulating causes, 

 and to what extent they will respond if these be supplied under 

 the water. 



The results of the experiments so far seem to justify the 

 assertion that the stimulus to the development of the water form 

 in the plant under discussion is not involved in the light rela- 

 tions, in the nutritive conditions, temperature, the gaseous con- 

 tent of the water, nor contact stimulus. The only factor which 

 IS constant in all cases where the w^ater form develops is the 

 checking of transpiration and the consequent increased amount 

 of water in the protoplasm. When the protoplasm of the pri- 

 iTiordial cells is in that condition of dilution which accompanies 

 the absorption of a large amount of water, the nature of the 



