THE BRYOPHYTA 113 



unicellular ; the cell- walls of the older hairs have a 

 brownish colour, but give cellulose reactions. 



The growing-point of each branch lies at the base of a 

 depression between the lobes, just as in the prothallus 

 of a Fern. The growth here goes on by means of a 

 single large apical cell, which cuts off segments both at 

 its sides and base. The former build up, by their 

 subsequent growth and divisions, the lateral parts of the 

 thallus, while the basal segments are chiefly concerned 

 in forming the midrib. The tissue derived laterally from 

 the apical cell grows more rapidly than the apex itself, 

 which consequently always lies in a recess of the 

 margin. 



The branching of the thallus, which as we have seen 

 may take place very freely, is dichotomous, the original 

 growing-point giving rise to two. The way this happens 

 is that a new apical cell is formed from one of the 

 lateral segments, and then both the apical cells go on 

 growing on their own account. The growing-point is 

 surrounded by short glandular hairs, which secrete 

 mucilage and so help to prevent the delicate tissues of 

 this part from drying up. 



We see then that the thallus of Pellia is both in 

 external form and internal structure a very simple 

 organism, bearing no resemblance to any of the plants 

 hitherto considered, so far as their asexual generation is 

 concerned. There is, on the other hand, a very marked 

 agreement with the prothallus of a Fern in form, struc- 

 ture, and general mode of growth. In fact, as we shall 

 find, Pellia and a Fern stand on nearly the same level as 

 regards their sexual generations, though the sporophytes 

 of the two are absolutely different. 



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