HELIOTROPISM 



469 



that although an excitation always follows a perception (direct excitation) it does 

 not follow that every excitation must be the direct consequence of a perception 

 occurring in the region concerned, since excitations may also be transmitted 

 (indirect excitation). We may therefore conclude from this experiment that 

 these two types of excitation are fundamentally distinct processes, for it is 

 only after indirect or transmitted and not after direct excitation that a reaction 

 occurs in the case of the seedlings of the Paniceae. Since, however, we observe 

 that a curvature follows in the young cotyledon also after a direct excitation, 

 we may take it as certain that the phenomena of excitation in the cotyledon 

 are in all cases identical with those in the hypocotyl, and that the non-appearance 

 of curvature in the cotyledon in its later developmental stages is solely due 

 to the cessation of growth. 



The phenomena just described as occurring in young plants of Setaria are 

 comparable to those which are seen in the majority of Gramineae (i. e. Poaeoideae). 

 In them no hypocotyl is developed, and the cotyledon attains very considerable 

 dimensions. It is sensitive to unilateral illumination throughout its entire 

 length, but the excitation, and also the reaction, is most vigorous when the apex 

 is subjected to unilateral illumination. This conclusion may be arrived at from 

 a study of several facts. The course of ordinary heliotropic curvature supports 

 this view. If we consider this pheno- 

 menon in Avena (Fig. 149), we note 

 that that curvature begins just below 

 the apex (b), but after 3^ hours (c), the 

 whole organ, has become affected ; later 

 on the apical region (after several oscilla- 

 tions forwards and backwards, probably 

 of an autotropic nature) again becomes 

 straight and the curving is localized at 

 the base, the radius of curvature being 

 at the same time reduced (d). If we 

 next attempt to determine the distri- 

 bution of growth in the seedling, we 

 find that the maximum is somewhere 

 about 5 to 10 mm. from the apex, and 

 that from this point upwards it be- 

 comes reduced very rapidly, but that, basally, the decrease is quite gradual. 

 The heliotropic curving does not, therefore, begin in this case at the zone of 

 greatest growth, but in a region where growth is very feeble, and hence it follows 

 that the excitation must be greater at the apex than lower down, since, if the 

 excitation were equally great throughout, the curvature must obviously start 

 in the region of most vigorous growth. We may arrive at the same conclusion 

 with regard to the distribution of heliotropic sensitivity by entirely different 

 means. Heliotropic curvature appears in Avena if we illuminate the whole 

 cotyledon on one side or only its base or only its apex, but the results are not 

 identical in the three cases. The most remarkable result is obtained when 

 we arrange, by means of a sheath of black paper of the form represented at 

 Fig. 150, /, that the light falls on the upper end of the seedling from the right 

 only, and on its base from the left. After I J hours the parts exposed to the light 

 have bent in the form of a double bow, and the whole structure takes the form of 

 an S. After five hours (Fig. 150, //, ///) the excitation transferred from the apex 

 downward has annulled the tendency of the base to bend to the left or even 

 transformed it into a reverse tendency ; in other words the transmitted ex- 

 citation prevails over that locally developed. The apical zone, specially affected 

 by the light, is very limited in extent, being at most 3 mm. long. From these 

 experiments it follows that the excitability of the base of the seedling (as measured 



a 



Fig. 149. Heliotropic curvature in Avena. After 

 ROTHERT (from DETMER'S Practical Botany), ij nat. 

 size, a, at the commencement of the experiment ; 

 b, after ij hours; c, after 3j hours; d, after pj hours. 



