CoGNiTic State of Energy 123 



(Chap. XV) to man, the pigmented eye-spots or eyes are anterior 

 structures that develop embryologically after the chemotactic 

 or olfactory and oral areas. 



The geotactic region in plants is the basal or fixing region. 

 In the free-swimming cells or zoospores of the unicellular, 

 as well as the multicellular, algse, the clear anterior ciliated 

 end that is devoid of chloroplasts is the fixing region. This 

 seems to be determined, in part at least, by molecular con- 

 stituents of the protoplasm, that are photophobic. But, 

 combined mth these or aided by them, there evidently is a 

 definite set of constitutent molecules that show a positively 

 geotropic and geotactic irritability. In the formation of em- 

 bryo plants from the spore also, amongst the Hepaticae, the 

 Musci, and the Filices or ferns, the first division wall laid 

 do^Ti nearly always serves to demarcate a colorless geotactic 

 cell from one that contains chlorophyll, and that is both apogeo- 

 tropic and heliotactic. 



Thanks to the experiments of Darwin, Czapek, and succes- 

 sors, we now know also that the growing tip of phanerogamic 

 roots is geotactic, and this over a neatly restricted area. Fur- 

 ther the propagation backward of a stimulus from the tip to 

 an older root area that is geotropic indicates that there is a 

 definite linking up of molecules, whether after the visible man- 

 ner described by Nemec may be left open for the present. 

 These seem so to affect the protoplasmic sacs of the geotroj)ic 

 cells that they lose water on one side and become more turgid 

 on the other, so as to cause bending in the root as a whole. 



In multicellular animals the geotactic sense appears to be so 

 intimately blended and associated with the gyratory that a 

 common sense-center for both is alone suggested by the move- 

 ments executed under natural and experimentally modified 

 states. But the definite geotropic responses secured by Loeb 

 for some actinians and echinoderms (60: 57) are proof that 

 specialized sense-centers exist in these. The constant effort 

 also, on the part of most animals, to assume a definite 

 satisfied position in relation to gravity is strong suggestion 

 alike of a geotropic sense and of geotactic centers. But our 



