DIFFERENTIATION OF ORGANS IN THALLOPHYTA 21 



no hesitation in calling the leaf-like organs which we find in many 

 Thallophyta 'leaves.' Some Florideae have throughout a differentiation 

 analogous with what is found in the higher plants, for example, Polyzonia 

 jungermannioides which is represented in Fig. 17; examples too are not 

 wanting in different developmental series of the Phaeophyceae, such as the 

 Laminariaceae and Fucaceae. 



The way in which this differentiation into shoot-axis and leaf has 

 come about varies considerably. It will be briefly shown in the next 

 chapter how in the lower plants such a differentiation has been reached 

 by different paths from quite simple beginnings. 



III. 



FORMATION OF ORGANS AND DIVISION OF LABOUR 

 AMONGST THE LOWER PLANTS (THALLOPHYTA). 



A plant-body in which the shoot-axis does not exhibit differentiation 

 into stem and leaf is termed a thallus. The earlier writers called the 

 thallus a ' frond ' when it was flat and leaf-like, but this is a superfluous 

 term which has fortunately dropped out of use. The expression thallus, 

 which signifies nothing more than shoot, was first used by Acharius 1 in 

 describing the lichens, and subsequently it was extended to the Algae, 

 the Fungi, and the thallose liverworts. There is no sharp limitation 

 between a thallus and a leafy shoot as the examples which will be 

 presently noted clearly show. Endeavours have also been made to 

 establish the idea of the ' phytome ' in addition to that of the thallus. 

 By this term Naegeli designated the plant-body of unicellular plants, 

 and of such as consist of entirely similar cells whether unbranched or 

 branched, provided that in the latter case the branches be always similar 

 to one another and to the mother-organ. The distinction between the 

 phytome and the thallus was based on the fact that the thallus could 

 produce ' trichomes.' The distinction is altogether superfluous. The idea 

 underlying the expression many-celled ' phytome ' is included in that of 

 colonies or coenobia, which will be described presently, and it is impossible 

 to speak of ' trichomes ' among the Thallophyta in the same sense as 

 we speak of them amongst the higher plants. The organs which have 



1 Acharius, Lichenographia universalis. Gottingae, 1810, p. 3 'In omni Licheno complete 

 duae . . . ;ese offerunt partes, quarum una corpus ipsius Lichenis constituens thallus a me 



dicitur . ,' 



