120 Causes and Course of Organic Evolution 



is sho^Ti, till it becomes a striking and inherent characteristic 

 in nearly every one of the higher species. The question may 

 well be asked then: Are there special chromatin or sense per- 

 ceptive centers for perception and focusing up of the heliotactic, 

 geotactic, and other stimuli? During the past sixteen years, 

 the writer has placed it before his students, as a working hy- 

 pothesis, that the different tropisms shown by living organisms 

 can alone be satisfactorily explained if we accept it that definite 

 plastids or energids show a special sensitivity and polarity 

 to environal stimuli. Such bodies he termed geoenergids, 

 helioenergids, etc. 



Thus when one placed, as he first did eighteen years ago for 

 this purpose, roots of pea or broad bean at right angles to 

 gravity, and after 12 to 24 hours removed, fixed, cut, stained, 

 and mounted sections of the now curved root in balsam, it 

 was found repeatedly that a much larger number of minute 

 stainable chromatin-like bodies were in the shorter and more 

 richly protoplasmic cells of the lower root surface than in the 

 longer, paler, and more vacuolated cells of the upper. More 

 recently Haberlandt seems also to have recognized these. 



The manner in which ciliated algoid and fungoid cells, as 

 well as plant spermatozoids, move by their clear anterior end 

 towards any area of chemotactic stimulation indicates either 

 that the protoplasm is specially chemotactic in that region 

 or even that some definite plastid or plastids may there be 

 located, unless indeed the frequent thickening at the base of 

 the cilia does not represent such a chemotactic center. 



We know, however, from the investigations of Meyer and 

 others, that leucoplasts are of wide occurrence in plant cells, 

 that they can transform sugar into starch, and may themselves 

 at times become chloroplasts. When we add to this that 

 chloroplasts can usually orient themselves in most algae and 

 in higher green plants to incident fight rays, there seems con- 

 siderable warrant for the view that chemoenergids, photo- 

 energids, and geoenergids have evolved amongst the higher 

 members of the simple algae in the order indicated, and have 

 persisted throughout the vegetable kingdom. 



