526 



LECTURE KXXI. 



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 through the lateral portion of a flat shoot of Marchautia polymorpha. o epidermis 

 of upper, w of lower side; p colourless parenchyma of lower side ; r A/ assimilatory cells containmg chlorophyll ; sp 

 stoma \ s s boundary between two areolae. 



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 gemmse 

 of Marchantia 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, t-he 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 



FIG. 342.— Vertical section of a ripe gemma of Marchatitta, carried through the two growing-points, a hyaline 

 cells which develop into root-hairs ; b cell filled with peculiar contents ; rfwing of recess in which the growing-points 

 e are situated tX iSg). 



on both the convex sides, which are destined, according to circumstances in e&ch 

 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 lowef 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, 'Uber'die Einwirkung des Lichtes auf dm Marchantkn- 

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



