HOFMEISTER, ON THE HIGHER CRYPTOGAMIA. 183 



gated ; as may be observed in Asplenium septentrionale 

 (PL XXIV, fig. 3). In most cases however this elonga- 

 tion does not occur. I have never seen it in Pieris aquilina, 

 Aspidium filioc-mas, Ceratopteris or Adiantum. In those 

 species where considerable elongation of the lowest cell 

 occasionally takes place, it seems to be caused by deficiency 

 of light. The first rootlet is produced during the con- 

 tinuance of the transverse division of the terminal cell of 

 the young prothallium : it has the form of a cylindrical 

 process developed from a protrusion of the membrane of 

 the lowest, more rarely of the second lowest, cell of the 

 prothallium. The rootlet very soon after its appearance is 

 separated from the original cavity of its mother-cell by a 

 septum convex towards the interior (PL XXIV, figs. 1 — 3). 

 After about the fifth transverse division of the apical cell 

 of the young prothallium, this cell divides by a longitudinal 

 septum. The two apical cells afterwards divide frequently 

 by transverse septa. In the cells of the second degree 

 which are thus formed transverse septa are produced. The 

 formation of these septa is however usually suppressed in 

 the two or three first-formed lowermost pairs of cells and 

 often also in one of the two next equal-aged cells (PL XXIV, 

 fig. 2). Thus the prothallium begins to be converted into 

 a cellular surface. At the same time the direction of its 

 growth is turned more and more from the light, so that it 

 soon assumes a position parallel to the surface of the 

 ground : when the light is powerful it adheres closely to 

 the earth.* 



The apical cells often divide also by longitudinal septa, 

 which diverge slightly from the longitudinal axis of the 

 prothallium, and which, like all septa which are produced 

 during the early growth of the organ, are perpendicular to 

 its surface. The prothallium has now four apical cells, of 

 which the two outer ones grow more rapidly in length than 

 the median ones, and immediately divide repeatedly by 



* The turning away of the prothallium from the light, by which the fore 

 edge of each prothallium is continually diverted from the source of light, was 

 first observed by Wigand (''Beitrage zur Botanik,' 1854, p. 35). His explana- 

 tion however is altogether erroneous ; he assumes that the upper surface of the 

 prothallium was drawn from the light. If this were so the prothallium must 

 turn itself to the light. 



