120 



PLANT STKUCTURES 



radial, in contrast with the dorsiventral thallus, referring 

 to the fact that it is exposed to similar conditions all 

 around, and its organs are arranged about a central axis 

 like the parts of a radiate animal. This position is much 



more favorable for the 

 chlorophyll work than 

 the dorsiventral posi- 

 tion, as the special 

 chlorophyll organs 

 (leaves) can be spread 

 out to the light freely 

 in all directions. 



It should be re- 

 marked that the gam- 

 etophyte in all groups 

 of plants is a thallus, 

 doing its chlorophyll 

 work, when it does 

 any, in a dorsiventral 

 position ; the only ex- 

 ception being the ra- 

 dial leafy branch that 

 arises from the thal- 

 lus of Mosses. From 

 Mosses onward the 

 gametophyte becomes 

 less conspicuous, so 

 that the prominent 

 leafy plants of the 

 higher groups hold no 

 relation to the little erect leafy branch of the Mosses, 

 which is put out by the gametophyte, and which is the 

 best the gametophyte ever does toward getting into a bet- 

 ter position for chlorophyll work. 



The leafy branch of the Mosses usually becomes inde- 

 pendent of the thallus by putting out rhizoids at its base 



Fig. 102. A moss {Bryvm), showing base of a 

 leafy branch (gametophore) attached to tlie 

 protonema. and having sent out rhizoids. On 

 the protonemal filament to the right and be- 

 low is the young bud of another leafy branch. 



— MULLEU. 



