918 EECORD OF CUIIRENT KESEA.ECHES EELATINa TO 



prothallium of ferns (i. e. the exclusive tendency of one side to 

 produce antlieridia and archegonia) is the result mainly of a diiference 

 in the supply of light, rather than of gravitation ; the organs of 

 reproduction being produced only on the shaded side. His con- 

 clusions were confirmed by the following observations on Ce7-ato])teris 

 tJialictroides. 



Prothallia of this fern are produced with ease by sowing the spores 

 on a nutrient fluid. When lighted from above, the prothallia expand 

 on the surface of the water, or raise themselves above it, turning one 

 side to face the light. Only the shady side produces archegonia, the 

 rhizoids springing either from it or from the marginal cells, and 

 growing downwards into the fluid. But when the light reaches the 

 prothallia from below, the result is quite different. They grow down 

 into the fluid to meet the light, bat curve, as soon as the true surface 

 begins to develop, so that one surface is placed at riglit angles to the 

 incident light. The conditions with respect to contact with the 

 substratum are therefore here the same on both sides, and yet the 

 archegonia, as well as those rhizoids which spring from the surface, 

 are produced on one side only, viz. the shady side. The rhizoids 

 also do not grow downwards, but spread themselves over the surface 

 of the fluid ; they are therefore not positively geotropic, but to a 

 certain extent negatively heliotropic. 



These prothallia are also instructive in another respect. The 

 rhizoids are, as is well known, produced especially at the basal end 

 of the cells. This occurs also in the parts of the prothallium which 

 grow obliquely downwards, proving that it is not gravitation that 

 determines their place of origin, but that it depends on the contrast 

 of base and apex of the prothallium. 



Development of the Prothallium of Salvinia.* — Prantl has 

 closely followed out the history of development of the prothallium 

 and reproductive organs of Salvinia natans, with the following results, 

 which are not in every respect in harmony with those previously 

 arrived at by Bauke.f In addition to the wall of the si^orangium, the 

 true spore is surrounded by a small-celled integument, the exospore 

 of Pringsheira, but more correctly called the epispore by Eussow and 

 Sachs. This is firmly attached to the thick yellow exospore, which, 

 on germination, becomes detached in three lobes from the apex of the 

 spore. At the same time the epispore is also burst into three lobes. 

 The ripe sj)ore always swims on the surface of the water, and is not 

 merely buoyed up by algfe, as Hofmeister supposed ; the entire 

 epispore serving as a swimming apparatus. Germination takes place, 

 under favourable conditions, about Christmas. 



The germination, development of the prothallium, fertilization, 

 and the development of the young plant until the " peltate leaf" has 

 attained its full size, take place in the dark, but with formation of 

 chlorophyll. The first cell-formations in the prothallium are 

 exceedingly difficult to follow. The meniscus-shaped anterior portion 

 of the spore is first of all separated from the large spore cavity by a 

 single basal wall, the " hyaline membrane " of Pringsheim. A young 

 * 'Bot. Zeit.,' xxx-vii. (1879) p. 425. f This Journal, ante, p. 753. 



