HEREDITY OF ACQUIRED CHARACTERS 199 



The proof has been found in experiments with Proserpi7iaca 

 palustris, a plant of the United States of the same family as our 

 water milfoil and mare's-tail [Halorageoi), This plant is am- 

 phibious ; when the leaves are in the air, they are completely 

 formed, being of a narrowly lanceolate type, with a serrated 

 margin ; but when submerged the plant bears leaves consisting 

 of the midrib and lateral veins only, so that it acquires a 

 "pectinate" or "comb-like" form. 



In order to ascertain the actual cause of the change of form, 

 Mr. McCullum experimented in various ways, finding that even 

 a saturated air produced the submerged form ; and that by 

 eliminating all other probable or possible causes, water only 

 could be credited with inducing the submerged type of foliage. 

 He thus writes : — 



" The essential feature common to the water and moist air 

 is the inhibition of transpiration and the consequent choking 

 of the cells and diluting the protoplasm with water. This can 

 be tested by growing the plants entirely under water and at the 

 same time drawing the water out from the protoplasm ; or in 

 reality causing evaporation or transpiration by means of high 

 osmotic pressure. Plants producing water-leaves were placed 

 in nutrient solutions of a strength not quite sufficient to plas- 

 molyze them : also in very dilute solutions made up to the 

 same osmotic pressure with KCl and also Ca(N03).2. These 

 were allowed to evaporate down, becoming gradually stronger. 

 When they reached a strength of solution equal to about half 

 N salt solution the water leaves ceased to form, ajid the air 

 type of leaf appeared. " ^ 



Here, then, we have the desired experimental evidence that 

 the dissected form of foliage found in so many submerged 

 plants is actually caused by the superabundance of water 

 saturating and weakening the protoplasm. 



Now let us return to Ranunculus heterophyllus or R. tricho- 



' " On the Nature of the Stimulus causing the Change of Form and 

 Structure in Proserpinaca palustris" Bot, Gaz., vol. xxxiv., p. 93. 



