614 SENSORY SYSTEM 



similarly, it is opposite portions of the epidermal layer that must 

 experience the greatest differences of light-intensity. It is nevertheless 

 highly improbable that definite epidermal or cortical cells, or groups of 

 cells, distinguished by special histological features, are set apart for 

 the purpose of light-perception, in the case of ordinary stems ; at any 

 rate, there is at the present time no evidence in favour of the existence 

 of any such specialisation. 



Charles Darwin discovered that the apices of the positively helio- 

 tropic cotyledonary sheaths of certain Grass-seedlings (Avcna sativa, 

 Phalaris canariensis, Panicum miliaccum, Sctaria viridis, etc.) are 

 particularly sensitive to photic stimuli ; more recently, Kothert has 

 confirmed Darwin's observations by means of very elaborate and 

 painstaking investigations. 316 The heliotropic stimulus travels from the 

 apex towards the proximal end of the cotyledon, and in the case of the 

 Paniceae even into the hypocotyl which is itself quite insensitive 

 to direct illumination and thus induces, or at any rate, accelerates, the 

 heliotropic curvature of the basal region. In Avcna sativa, the 

 extreme apex of the cotyledon (comprising a zone from 1-1 \ mm. in 

 length) is specially sensitive ; behind this region heliotropic sensitive- 

 ness rapidly decreases, being no greater 3 mm. from the tip, than 

 it is at the actual base of the organ. 



The tip of such a Grass-cotyledon might well be termed a 

 heliotropic sense-organ; it has already been remarked (p. 608) that 

 the same region of the cotyledon represents or rather contains a 

 gravitational sense-organ as well. A priori, it seems somewhat unlikely 

 that statocysts should also act as photic sense-organs. In the coty- 

 ledonary apex of Phalaris canariensis, Panicum miliaceum, Meusine 

 inclica, etc., all the elements of the fundamental parenchyma are developed 

 as statocysts ; assuming that there is any division of labour at all, the 

 only cells available for photic perception are the elements of the outer 

 epidermis, which are entirely devoid of starch (the inner epidermis need 

 hardly be taken into consideration). That the faculty of light-perception 

 is confined to the epidermis, seems all the more probable in view of the 

 fact that the underlying statocyst-tissue contains numerous air-spaces of 

 considerable size, so that the light which penetrates beneath the epidermis 

 must be repeatedly deflected from its original direction by reflection and 

 refraction. Nevertheless, the epidermis displays no special histological 

 features that could be regarded as adaptations in connection with light- 

 perception. At the extreme tip of the cotyledonary sheath, the cells of 

 the epidermis often, though by no means always, assume a palisade- 

 like shape {e.g. in Avcna sativa, Phalaris canariensis, etc.). All the epi- 

 dermal elements of the specially sensitive region are provided with 

 dense protoplasmic contents, a feature which is on the whole very 



