164: ROBERT PAULSON ON THE 



travelled, for all three forms appear in certain of tlie larger families, 

 as in the Parmeliaceae, the Physiaceae and the Cladoniaceae. 



Eeproductive organs occur upon the thallus. On the fruticose 

 form they are produced at the end or side of the branches ; on 

 the foliose and crustose forms they occur upon the upper surface 

 either in the centre or on the edge of the thallus. These organs 

 are sometimes disc-shaped and are known as afothecia, or they are 

 flask-shaped bodies called the perithecia, the latter often being 

 immersed in the thallus or even in small depressions of the rock. 

 The pits have been caused by the action of the lichen upon the 

 stone. 



Lichens are divided into two great series according to the form 

 of the reproductive organs ; those with the open disc (apothecium) 

 belong to the Gymnocarpeae, and those with more or less closed 

 perithecium are included in the Pyrenocarpeae. It is not proposed 

 to describe the microscopic structure of the reproductive organs 

 just mentioned, for the time at our disposal will not admit of this ; 

 but a short descripton of a non-vegetative method of reproduction 

 of the green cells, described for the first time quite recently, will 

 be given later on (3). 



Lichens may be regarded from quite another standpoint ; they 

 are when moist either gelatinous, like the well-known alga 

 Nostoc, or non-gelatmous. These respectively represent two 

 entirely different types of structure as regards the thallus, the 

 former having the green cells, gonidia, scattered evenly through 

 the thallus, while in the latter they form a definite layer closely 

 under the upper surface. 



The lichen selected for more detailed description is Cladonia 

 digitata, a dorsiventral, non-gelatinous plant. 



It was in 1868 that Schwendener announced his view that 

 a lichen consists of two distinct organisms, a fungus and 

 an alga, growing in close relationship (5). In a remarkably 

 short time, less than a decade in fact, this view was generally 

 accepted. 



The fungus consists of fine septate hyphal threads of from 3 

 to 4 microns in diameter, these being greatly modified according 

 to the position they occupy in the thallus. The gonidium (algal 

 cell) of Cladonia digitata is spherical in shape and bright green 



