RELATIONSHIPS OF SYMMETRY 



(1) Cetraria islandica possesses a dorsiventral flat or upwardly-concave thallus 

 which grows somewhat obliquely upwards. In many of the branches when they 

 grow upwards to a considerable degree the edges of the thallus may become 

 in-rolled and united so as to form a tube, and this is the rule in the variety known 

 as crispa. This kind of thallus-branch always has a radial structure inasmuch as 

 the green algal cells are equally distributed in it, whilst in the dorsiventral branches 

 these algal cells lie chiefly on the upper side. The 



advantage to the plant from a mechanical point of 

 view of this in-rolling of the edge of the thallus hardly 

 requires mention; it is evident that a thallus-surface 

 can hold itself erect more easily when in-rolled than 

 when spread out. 



(2) In Thalloidima vesiculare, club-like out- 

 growths arise from the thallus which recall many 

 of the lower fruticulose Algae, and in the beard-like 

 lichens the thallus develops from the beginning in 

 this way l . 



(3) In some lichens the 

 stalk of the ascocarp, called the 

 podetium, is strongly developed 

 and of importance as an assimila- 

 tion-organ, for example in Pycno- 

 thelia, Glossodium, and others 2 . 

 If we suppose these podetia to be 

 branched, we should obtain the 



FlG. 31. Cladonia cocci- 



fera. A cup-like podetium . , r , . 



bearing marginal stalked Special lOrill Observed in many 

 apothecia rises from the 

 horizontal thallus. 



(Lehrb.) 



species of Stereocaulon, where 



the primary thallus, composed 

 of granules and scales, gives rise to much-branched 

 fruticulose structures, the stronger branches of which 

 bear at the end the fructifications, whilst the weaker 

 portions remain sterile, have limited growth, and serve 

 entirely as organs of assimilation. It is quite the 

 same in the genus Cladonia to which so many species 

 belong 3 (Fig. 31). The starting-point of the develop- 

 ment is here also a dorsiventral flat thallus upon 

 which the fructifications sit directly in the simplest 

 cases, whilst in others the fructifications are stalked and 

 branched. Amongst the Cladonieae two chief forms 

 are found which are connected by intermediate states. 

 These are the fruticulose form and the scyphiferous form. In the latter an enlarge- 



FlG. 32. Cladonia verticillata. 

 Natural size. 



1 See Reinke, Abhandl. iiber Flechten III, in Pringsh. Jahrb. xxviii. p. 105. 



2 See figures in Reinke, I.e. 



3 See Krabbe, Entwicklungsgeschichte und Morphologic der polymorphen Flechtengattung 

 Cladonia. Leipzig, 1891. 



