OF ARTS AND SCIENCES. 85 



Reinke also distinguished what he terms Rundtriebe and Flachtriebe, 

 which are interesting as showing the close connection of the apical 

 and marginal forms of growth. The Rundtriebe and Flachtriebe 

 resemble each other closely, both having the apical form of growth, 

 differing only in that the former are round, while the latter, as the 

 name implies, are more or less flat. The change from the Flach- 

 triebe to the Breittriebe, as Reinke terms the expanded form of 

 the frond, is accomplished by a change in the mode of growth. Ac- 

 tively growing marginal cells, like those already described, become 

 differentiated from the cells bordering the edges of a Flachtrieb, 

 and the apical cell is obliterated. A very characteristic feature of the 

 expanded form of Padiua, and one that should not be passed over, 

 is the hairs which beset both surfaces of the frond. They occur in 

 prominent and regular concentric lines, and are developed from the 

 outgrowth of the superficial cells. 



In Taonia atomaria, Ag., we have perhaps a more typical form of 

 those genera where the growth is marginal. For a detailed descrip- 

 tion one should refer to Reinke's paper already alluded to. A few 

 words regarding it will suffice here. In the structure of the frond it 

 differs mainly from Padina in two features ; the edge of the frond is 

 not rolled inwards, as in the latter genus, and the hairs, instead of be- 

 ing in definite parallel lines, are in more or less irregular zones. Be- 

 sides Taonia atomaria, Reinke describes another Mediterranean form, 

 with a flat and not incurled margin, which he considers to be Zonaria 

 parvula, distinguishing it, however, from the so called Zonaria parvula 

 of some older writers, now included in Aglaozonia reptans, one of the 

 Cutleriaceae. The frond of Z. parvula is thin, flat, more or less irreg- 

 ularly expanded, and on its under side beset with rhizoid-like hairs, 

 which attach it to the substratum. The growth is by marginal cells, 

 as in Padina, but it resembles Taonia in having the edge of the frond 

 flat. Usually the thallus is but three cells in thickness, consisting of 

 a single-layered, small-celled cortex on each side of the frond, and of a 

 large-celled internal portion. The latter layer, while it is usually only 

 one cell in thickness, may in places become divided mto several layers. 

 The growth of the frond from the marginal cells is very much as 

 in Padina, except that both of the cortical layers are formed simul- 

 taneously. 



The reproductive organs, while they differ considerably in their 

 arrangement in the various genera, are much alike in structure. They 

 are all outgrowths of cortical cells. Tlie development of the tetra- 

 spores is as follows. A group of cortical cells becomes considerably 



