TUB PLANT BOt>T. 61 



entiation into members is not so marked as in the higher, 

 and in passing downward in the vegetable kingdom groups 

 are reached in which it is inappreciable, and finally in 

 which it is entirely wanting: such an undifferentiated 

 plant-body is called a Thallome, and may properly be re- 

 garded as the original form, or prototype. 



117. Thallome. — The simplest thallome is the single cell; 

 this, though generally rounded, is in some cases irregu- 

 larly extended into stem-like or leaf-like portions, which 

 may be regarded as, to a certain extent, foreshadowings or 

 anticipations of the members of the higher plants. Plants 

 composed of rows of cells or cell-surfaces frequently show 

 no indication whatever of a division into members; but in 

 some cases there is a little differentiation, which, though 

 not carried far enough to give rise to members, is the same 

 in kind. 



118. In the larger seaweeds there is sometimes so much 

 of a differentiation that it becomes difficult to say why 

 certain parts ought not to be called members. Structures 

 of this kind are instructive, as showing that the passage 

 from the thallome plant-body to that in which members 

 are differentiated is by no means an abrupt or sudden 

 one. 



119. Caulome. — By this general name we designate all 

 axial members of the plant. In the more obvious cases the 

 caulome is the axis which bears leaves (foliage), and in this 

 form it constitutes 



(1) The Stem ; branches are only stems which originate 

 laterally upon other stems. 



The other caulome forms are: 



(2) Runners, which are bract-bearing, slender, weak, and 

 trailing. 



