228 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



a more accurate research into the matter. We have here to distinguish two 

 things : first of all the appearance of the dorsiventral thallus-structure, 

 and then the outgrowth of the hair-roots for Wiiich special cells are 

 laid down on the gemma. The side upon which the light falls is 

 always the dorsal surface, even if the gemma float in water and be 

 illuminated only from below ; in such circumstances the surface directed 

 downwards is the dorsal surface. The outgrowth of the primordia of 

 the hair-roots is also influenced, although not so exclusively, by light, if 

 it be sufficiently intense. In darkness the gemmae do not usually 

 develop, and if they do they form no hair-roots, or only a few. 

 Zimmermann found upon twelve gemmae illuminated from below 

 thirty-nine hair-roots on the shaded side and four upon the illuminated 

 side. With reference to the influence of gravity and of contact with 

 solid bodies upon the outgrowth of the hair-roots I must refer to 

 Pfeffer ; the gemmae may produce hair-roots on both sides, whilst 

 the thallus which develops out of them is always dorsiventral, and 

 the dorsiventrality is fixed after the influence has lasted three or four 

 days, although by that time the anatomical construction has not yet 

 appeared. 



Similar behaviour is exhibited, according to Leitgeb, by the germ- 

 plants of different liverworts. The ' germ-disc ; of Marchantieae, for 

 example, is not at first dorsiventral ; its dorsiventrality depends upon light 

 which determines the side which is to be dorsal, and that which is to 

 be ventral. Once the dorsiventrality is established it is permanent, as 

 in the above-mentioned instance *. 



Dorsiventral prothalli of ferns behave in quite a different manner. 

 In them the dorsiventrality shows itself by the formation on the under 

 side of a cushion of tissue from which the archegonia and hair-roots 

 take origin. The dorsiventrality is here at any time reversible 2 . If 

 a prothallus floating on water be illuminated from below, the new 

 archegonia and hair-roots develop upon the upper side, that which 

 is turned away from the light. If a prothallus be cultivated on a 

 klinostat with the axis of rotation vertical and if the illumination be 

 lateral, it forms archegonia only on one side ; perhaps this occurs because 

 there is not an equally strong illumination on both sides, or it may 

 be because a bilateral construction is here impossible from ' inner ' 

 causes 3 . Upon the advantage to the plant of the archegonia arising 



1 It is clear that as it is a mere accident which side of the gemma is turned upwards, and as there is 

 not always a definite disposition of the germ-disc, it is an advantage that light has a determining 

 influence in the further differentiation. 



2 Leitgeb, Studien iiber die Entwicklung der Fame, in Sitzungsb. d. Wiener Akad. d. Wissensch. 

 Ixxx (1879). 



3 Occasionally prothalli occur bearing archegonia and hair-roots on both surfaces and over 

 a considerable extent of them. 



