GENERALIZED FORMS. 135 



for members coordinate with those mentioned above, as they 

 are only parts of a unit, instead of members of a body ; they 

 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. In the larger algae there is sometimes so much of 

 a differentiation that it becomes difficult to say why certain 

 parts ought not to be called members. Caulome and phyl- 

 lome, at least, are strongly hinted at in the Fucaceae, and 

 in this group, although the term thallome is applied to the 

 plant-body, it must be admitted as not fully applicable. 

 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. 



171. Mutual Relations of Thallome, Caulome, and 

 Phyllome. The caulome is the phyllome-bearing axis of the 

 plant, and phyllomes are the members developed upon the 

 caulome. The two have a reciprocal relation, and in no 

 case is the one present without the other. The definition of 

 the one involves that of the other. Both are derived 

 directly from the thallome, and that differentiation which 

 gives rise to one necessarily produces the other. The differ- 

 entiation of thallome into caulome and phyllome is simply 

 a lobing and contraction of the marginal portions into sepa- 

 rable phyllomes, and a rounding and contraction of the 

 central or axial portion into a caulome. 



172. 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. 



* From the Greek xniv.oS, stem. 



