520 LECTURE XXXI. 



itself must be completely reversed, i. e. the side marked o would become the 

 lower side. It is to be mentioned moreover that the Marchantia shoot produces 

 on the under or shaded side two series of membranous leaves near its middle 

 line, and very numerous root-hairs. That it is possible to shape this shoot at 

 will during continuous illumination from one side so that its organic light-side 

 is turned downwards or upwards, forwards or backwards, is greatly owing to 



FIG. 341.— Vertical section tlirouyh tlie lateral port 

 of upper, u of lower side ; / colourless parenchyma of I 

 stoma ; i- s boundary between two areolse. 



the circumstance that neither the spores nor the gemmae from which the shoots 

 arise are differentiated dorsi-ventrally. Thus with the beginning of the growth of 

 the new shoot the influence of the light upon it can come into effect at once, 

 without being modified by previous influences. In this connection the gemmae 

 of IMarchantia have already been investigated. They arise from papillae in small 

 cup-shaped receptacles on the light side of older shoots, and constitute later 

 approximately lenticular bodies, the two convex sides of which are organised quite 

 alike and are sensitive to external influences in exactly similar degree. As shown 

 on the transverse section (Fig. 342) through such a gemma, certain cells a exist 



> growing 



■points, a hyaline 

 :he growing-points 



on both the convex sides, which are destined, according to circumstances in each 

 case, to grow out into root-hairs ; if both the surfaces of the gemma are equally 

 illuminated those grow out which are able to follow directly the influence of 

 gravitation, i.e. those of the lower side, whereas those of the upper side are 

 prevented from developing. The influence of the light however is stronger than 

 that of gravitation, since, as Zimmermann has shown \ if the lower surface is 

 illuminated it is chiefly the roots of the shaded upper side which develope. 



' On this point cf. Zimmermann, ' Über die Einwirkung des Lichtes auf den Marchantien- 

 thaliits' (Arb. des bot. Inst, in Wzbg. B. II, p. 665). 



