418 BOTANICAL GAZETTE [JUNE 
is not essential to the positive response; but that the more 
characteristic feature in both organs is the perception. The 
clearness of the distinction between perception and response 
demands that we discriminate in the use of words to name the 
two processes. Tropism itself is the disposition to respond by 
turning or bending, and has no necessary reference to the place 
or manner of the perception of the stimulus, more than that the 
two processes be some way connected in space and time. The 
young hypocotyl bends downward, and is therefore prosgeotro- 
pic. For the act of perception Czapek proposes the word 
‘‘aesthesis.’’*3 The root tip is prosgeoesthetic ; it cannot pos- 
sibly be geotropic because it cannot actively turn. The very 
young hypocotyl seems not to be directly irritable at all by 
gravity, but when it becomes so it always bends upward, that is, 
it is apogeoesthetic. With perception and execution, and 
transmission when the two other processes do not occur in the 
same place, the performance is complete. Czapek’s introduc- 
tion of a ‘“Reflex-centrum™ as a potential link between the 
organ of perception and that of response, and as a seat of inter- 
pretation and decision, seems to me unnecessary,’ and is with- 
out any empirical support. It is simpler and hence preferable 
to suppose that the organ of perception itself in the act of perceiv- 
ing determines the direction of the response, whether or not it 
itself executes it. The manifestations of irritability in plants, 
like those of instinct in animals, are very short-cut psychic 
processes, and constantly dispense with steps which would be 
necessary to the attainment of the same end by the exercise of 
intelligence. On many grounds, the perception should be 
regarded as the characteristic and decisive though invisible 
feature of the entire phenomenon in plant irritability. 
There is supposed to be a considerable number of instances 
13 Weitere ce zur Kenntniss der geotropischen Reizbewegungen. Jahrb. 
f. wiss. Bot. 32: se 1898. 
Zc. 294. ropisms were called “ Redccieacugen” by Oltmanns (Ueber 
die RE ah Bewegungen der Pflanzen, Flora 75: 265. 1892), but the elabo- 
ration of the idea is Czapek’s 
4S NOLL: Ueber Gunaieas Jahrb. f. wiss. Bot. 34: 492-496. 1900. 

