530 I'RINCIPLES OF GENERAL PHYSIOLOGY 



independently by Haberlandt (1900) and by Nemec (1900). It assumes that each 

 cell of the sensitive tissue corresponds to a statocyst of the animal. In the plant 

 cell, the statoliths are usually starch grains, which fall and form a little heap on 

 the lowest part of the cell. The precise part of the cell thus affected depends on 

 the position of the root or stem as regards the vertical line. In plants in which the 

 statoliths consist of starch, exposure to cold causes them to be used up and 

 the reaction to gravity is abolished until more are formed in warmth and light. 



The direction of light is appreciated by the leaves of plants, as shown by their 

 setting themselves at right angles to it. Haberlandt (1909, p. 557) points out 

 that, in many cases, the outer ends of the epidermis cells are of a vaulted shape, 

 so that parallel rays of light would be brought to a focus somewhere near the inner 

 ends of the cells. If the axis of the cell is directed towards the light, the middle 

 of the base of the cell is most brightly illuminated, and it is to be presumed that 

 when the brightest part moves to one side or the other a reaction takes place in 

 the stem, the result of which is to bring the bright spot to the centre again. In 

 other cases the cuticle is formed into a lenticular shape. Haberlandt has shown 

 photographically that the light is actually brought to a focus on the inner ends of 

 the cells by these arrangements. 



The articles on sense organs in plants by Haberlandt (1904 and 1909) will be found of 

 interest. 



SUMMARY 



The finer the differences between external forces which an organism is able to 

 appreciate, the better equipped is it to make use of or to defend itself against 

 these forces. 



Nerve fibres themselves are not sufficiently easily stimulated by these forces, 

 except in cases where the latter are actually injurious and damage the structures 

 of the organism. There are, in fact, free nerve endings in the skin for the 

 appreciation of such nocuous stimuli. 



A mechanism of some sort is therefore necessary to magnify the various minute 

 forces acting on the organism, so as to produce a force of sufficient magnitude to 

 set up a propagated disturbance in nerve fibres. Such mechanisms may be of 

 different kinds, since nerve fibres are excitable by electrical, mechanical, chemical, 

 and other stimuli. These mechanisms are the " receptors." 



A primitive kind of chemical sense, allied to taste and smell, seems to be 

 one of the first developed. Touch receptors, to appreciate delicate contact, would 

 also be of early formation. 



Events occurring in the organism itself, as well as those of the external world, 

 require to make their existence known to the nerve centres. Hence we have 

 intero- and extero-ceptors. Amongst the former are the proprio-ceptoro, l>v which 

 an organ under the influence of excitation from the centres gives information of 

 its state of activity to the centres themselves. 



The distance receptors, such as the eye, ear, and, to a certain extent, those 

 for smell, are the most important in the development of the highest intellectual 

 qualities. 



Since nerve disturbances are all of identical nature, whatever be the kind 

 of external energy which acts on the receptor organ, it is clear that the ditVerence 

 between sensations derived say from the eye and the ear, must lx> due to the 

 arrangements of the nerve centres, the "analysers." This is Miiller's "law of 

 specific senserenergies." A nerve fibre of special sense, ho\\e\er excited, alwavs 

 gives rise to the same sensation. 



A receptor organ differentiated for a particular kind of stimulus, differs from 

 other receptors, in that it is sensitive to very small stimuli of the appropriate 

 kind, which would be far below the limit of appreciation by a receptor adjusted 

 for another kind of stimulus. The amount of light energy required to excite the 



