Evolution of Plants - 311 



As would seem to be true from palseontological evidence, 

 the specifically numerous and varied family of the Diatoma- 

 cese originated at a comparatively recent date from organisms 

 intermediate between the two last divisions. In the frequent 

 retention of a single greenish yellow to yellow-brown chromat- 

 ophore, "^dth one or more pyrenoid centers, in the division 

 of the chromatophore amongst some to form two or more 

 variously expanded plates, in the gradual silicification of the 

 cell-wall to a degree that is at times bafiling in its minute com- 

 plexity, and in its present geographical distribution, the group 

 is highly suggestive. 



It now includes 148 genera and about 3800 species. But a 

 detailed examination of these, alike as regards their chromat- 

 ophores, the shape and relative complexity of the wall struc- 

 ture, as well as their present distribution in fresh and salt 

 water, reveals interesting features that appear to have an 

 important bearing on their evolutionary history. 



The entire group has been subdivided, in many recent treat- 

 ises, into the Centricse and the Pennatse. Now, if we accept 

 this as an approximately natural classification, the latter 

 include 38 genera with 1244 species that are fresh- water and 

 44 genera with 1287 species that are marine. That is, dis- 

 tributionally, the genera and species are about equally divided 

 between the two habitats. The Centricse on the other hand 

 include 7 genera vnih 165 species that are fresh- water and 59 

 genera with 1046 species that are marine. Now, while some 

 have regarded the Centricse as the more primitive group, the 

 general structure and complexity of the shell, the number 

 and distribution of the chromatophores, as well as auxospore 

 formation methods, all indicate that the Pennatse are the more 

 primitive. Since they contain numerous existing fresh- water 

 species, and since the more complex Centricse are evidently 

 evolving much more as a marine than as a fresh- water group, 

 there seems considerable evidence for believing that the entire 

 series has wholly or mainly been derived from a common fresh- 

 water ancestry, but during the process of increasing speciali- 

 zation they are becoming increasingly marine. The entire 

 series constitutes another side-line of evolutionary progress, 

 that both in its fresh-water and marine species has failed to 

 evolve higher groups than itself. 



The widely disseminated class Ulvaceae has generally been 

 viewed as related to Tetrasporacese, though it represents a 

 great advance in cellular organization. But in both divisions 

 each thallus cell contains a single large chromatophore, with 

 one — rarely more — ^pyrenoid center. But, durmg evolution of 



