ON THE STUDY OF ADAPTATION IN PLANTS. 185 



tinue after the cause has ceased to operate. Thus 

 changes of climate or nutrition are followed by immediate 

 visible results,, but the results are but temporary, and dis- 

 appear when their causes are removed. He quotes the 

 alpine Hieracia as an example, which in the plains im- 

 mediately begin to produce numerous leaves and blos- 

 soms and assume a luxuriant habit, very different from 

 the original stunted plant, with its miserable and starved- 

 looking organs. 



In my opinion the external conditions, which produce 

 temporary changes, act in a twofold way : they can 

 prevent the appearance (or occurrence) of certain organs, 

 or they can influence the growth of those organs which 

 are originating in the special meristem. The first-named 

 effect becomes possible when the development of a plant 

 is not already definitely predestined from its first origin, 

 but is determined during the course of development. 

 The constitution of the individual Protoplasm certainly 

 gives direction to the course of development, but the actual 

 course followed depends on certain external factors. A 

 few examples will serve to explain this. Every one is 

 familiar with the fact that our Sagittarias (and also a 

 few other monocotyledons, aquatic and marsh plants) 

 produce two kinds of leaves, viz., ribbon-shaped, sub- 

 merged leaves and sagittate ones, which rise above the 

 water-surface ; the first-named forms occur during early 

 development when the plant grows in its usual state in 

 shallow water ; they represent, however, the only kind of 

 leaf which is formed when the plants grow in deep or 

 rapidly-flowing water. What could be more simple than 

 to assume that the one leaf-formation is an adaptation 

 to an aquatic habit, the other to an aerial one? And 

 yet this assumption would not be correct. For my own 

 researches have proved that the ribbon-shaped leaves 

 represent the original ones which are so widely diffused 

 among the Monocotyledons and that the sagittate ones are 

 clearly only a further development of the same. The 

 ribbon-shaped leaves occur also when the plant exists in 

 the air from the beginning of its life ; they have first to 



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