346 Rayjnond Pearl 



-obviously in many particulars very like a regulatory process as 

 thus defined by Driesch, This is perhaps more clearly shown 

 than anywhere else in the production of a series of generally "like" 

 parts by an organism. The present writer (Pearl, '07) has made 

 a special study of this process in the case of the plant Ceratophyl- 

 lum. In that paper it is shown that in the development of leaf 

 whorls and of branches, and in fact in the general morphogenetic 

 activity of the plant there is a progressive development of parts 

 towards a definite type. At the beginning of the production of a 

 series of like parts such as leaf whorls, the form of the whorl pro- 

 duced is quite variable and quite different from the type finally 

 established. Viewing the whole series of leaf whorls produced 

 ■ it may be said that the earlier whorls on any plant axis are "ab- 

 normal" in the sense that they are quite different from the type 

 which is finally attained. As more leaf whorls are formed they 

 come to conform closer and closer to the type of whorl which is 

 finally produced with great precision and constancy. Nov/ this 

 series of events is in some particulars like a regulatory process. 

 Beginning with the production of something which is "abnormal"* 

 (in the sense that it is different from the finally established type) 

 the organism finally produces the "normal" (i.e., a definite fixed 

 type of structure). In doing this the plant appears in a sense to 

 "profit by experience" in its morphogenetic activity. The form 

 of a whorl produced at any given point on a plant axis is a func- 

 tion in a mathematical sense of the previous developmental history 

 of the plant. 



It has been possible in the case of Ceratophyllum to work out 

 with a remarkable degree of precision the way in which this ap- 

 proach towards a definite type in the normal ontogenetic devel- 

 opment takes place. It was found, whether the character dealt 

 with was leaf production or branch production, that the change 

 as one passed from earlier produced parts towards those finally 

 produced was in accordance with a logarithmic curve. This 

 fact was characterized in the paper cited as the " first law of growth 



■* It is to be noted, however, that the "abnormality'' here is not the result of a previous "Stoning" as 

 la the case of a true regulation as defined by Driesch Qor. cit.) 



