t^A LECTURE XXXI. 



In both cases however they remain short; but it suffices to darken the parts mentioned 

 to make them elongate. Of course this takes place much more energetically if the 

 surrounding air is moist ; or best if the. parts of the stem concerned are surrounded 

 with soil, by which means they are darkened and kept damp at the same time. 



Much more energetic than in the case of the highly organised vascular plants is the 

 influence of gravitation, and still more so of light, on the formation of organs in the 

 Muscine» and other plants of simple structure. Since it is here possible to submit 

 plants in the very earliest stages of development, as they proceed from the spores or 

 gemmffi, to a definite influence of external forces, the effect begins before the plant 

 has yet had time to become organised in a manner unknown to the observer. It 

 is thus possible to allow plants of this category to grow so that one can determine 



beforehand and arbitrarily the disposition 

 of the organs one to another and towards 

 external influences. One of the most 

 interesting cases in this connection was, 

 first described by Leitgeb^ in the prothallia 

 of Ferns. As is already known to the 

 reader (p. 31) the germinating spore of a 

 Fern does not give rise forthwith to a Fern 

 again, but first to a plantlet of quite another 

 form — the prothallium. The germinating 

 spore first forms a germinal filament seg- 

 mented by transverse walls, which subse- 

 quently spreads out anteriorly, and at length 

 grows into a very thin leaf-like flat lamella, 

 which is deeply indented in front and 

 consists of only one layer of cells contain-* 

 ing chlorophyll : later, however, it produces 

 in the centre, behind the growing-point, 

 a cushion, several cells in thickness, from 

 which the female organs of reproduction 

 or archegonia arise in large numbers, 

 while on the posterior portion of the la- 

 mella, or at its margins, the male fertilising 

 organs or antheridia arise. Previously to 

 this, numerous cells at the hinder part of 

 the prothallus have grown out into long 

 root-hairs. The whole of this structure 

 now is most distinctly dorsi-ventral : root- 

 hairs, antheridia, and archegonia arise exclusively on the under side, if the prothallus 

 grows on a horizontal substratum and is lighted from above in the usual way. 

 Leitgeb has shown, now, that the dorsi-ventrality, as such, is in this case an effect of 

 light, and in the growing parts of the prothallus may be reversed at will if what 

 was hitherto the shaded side is illuminated, the sexual organs always being formed 



FIG. 340.— Prothallus of a Fern (Osmunda regalis\ seen 

 from below, a antheridia ; w root.hairs J v growing-point 

 (magnified). 



' Leitgeb, 'Utir Bilateralitdt der Prothallien,' in Flora, 1879, p. 317. 



