76 



ERYTHEA. 



cell seen in-longitudinal section at this stage, (Fig. 7) closely 

 resembles that of the typical fern-prothallium. 



Sooner or later the two-sided apical cell is replaced by the 

 type found in the fully developed thallus. This is probably 

 brought about as in other similar cases by the formation of a 

 transverse wall in the apical cell, dividing it into an inner 

 triangular (seen in longitudinal section) and an outer four- 

 sided cell. The latter is the definite apical cell. From this 

 at first three sets of segments are formed, two lateral and one 

 basal, but later four, as finally two sets of basal segments are 

 formed by walls inclined alternately toward the dorsal and 

 ventral surfaces of the thallus. This is the type of apical 

 cell found in the full grown thallus and is the same as that 

 of the Marchantiacese. Owing to the very similar appearance 

 both in form and size of the apical cell and its lateral seg- 

 ments, it is often difficult to say positively that there is but 

 a single initial cell. 



The first rhizoid does not usually appear until the young 

 thallus is already multicellular. It arises as an outgrowth 

 from the base of the germ- tube (Fig. 4, r.) but is not separ- 

 ated from it by a cell- wall. No new ones are formed for some 

 time, and these arise from the basal cells of the thallus. 



The further history of the development of the normal 

 thallus has been sufficiently described before and will not be 

 further considered here.^ 



The abnormal plants to which reference has been already 

 made appeared in a culture of other liverworts grown under 

 glass, so that the amount of moisture was greater than is the 

 case in their natural habitat. However it was afterward 

 found that plants growing spontaneously in unusually moist 

 or shaded places showed a similar tendency to an excessive 

 development of the vegetative parts of the thallus. So differ- 

 ent in appearance were these abnormal plants from the 

 usual form, that it was some time before their real nature 

 Avas understood. The most striking peculiarity was the 



5 Leitgeb, 1. c. Campbell, 1. c. 



