PALEOBOTANY — BERRY. 375 



whole protoplasm of these unicellular forms was virtually in con- 

 tact with the environment the degree of potency of modifying forces 

 must have been enormously greater than it was after the protoplasm 

 was inclosed in a barrier of cellulose, or was rendered relatively inert 

 by the formation of specialized systems of tissues. Thus these first 

 stages of plant evolution may not have required the eons of time 

 that at first thought seemed probable. During the Eophytic stage 

 of evolution the hypothetical prechlorophyllic phase passed grad- 

 ually into a second or chlorophyllic phase which may appropriately 

 be termed the Algal phase of evolution. The original bacteria-like 

 life forms obtained their energy from the geosphere and the hydro- 

 sphere. The formation of chlorophyll enabled them to make in- 

 creasing use of the atmosphere as a source of energy. This develop- 

 ment of chlorophyll, first as scattered granules and subsequently as 

 definite plastids, so that the sun's rays were utilized for photosyn- 

 thesis and the abstraction of carbon from the carbon dioxide of the 

 atmosphere, was perhaps the greatest forward step in the evolution 

 of life, second only in its importance to the origin of life itself. 

 That this second phase of plant evolution was very ancient is shown 

 by the traces of algae ill the Pre-Cambrian rocks and by various 

 collateral lines of evidence of plant activity, such as the vast amount 

 of carbon (graphite) in the Proterozoic schists, the presence of opal 

 silica clue to algal activity, and the large amounts of bedded iron ore. 

 Nearest to the bacteria are the Cyanophyceae or blue-green algae. 

 They are all tiny forms world-wide in their distribution, found in 

 the intercellular spaces of higher plants, on bark, leaves, and roots, 

 and forming, as the so-called gonidia, one of the two components of 

 the lichen thallus. Their cell nucleus appears to lack a limiting 

 membrane, their chlorophyll is not definitely organized into chloro- 

 plasts and they multiply entirely in a nonsexual way by fission. The 

 primitive epicontinental seas probably swarmed with Cyanophyceae 

 where the waters contained sufficient terrigenous sediments in sus- 

 pension to furnish the needed nitrogen, phosphorus, etc. Where 

 denitrifying bacteria rob the sea water of its nitrogen and precipitate 

 calcareous ooze, algae are not common. The alga stock subsequently 

 became diversified both in their vegetative and reproductive structures 

 and processes for the various marine habitats, and have changed but 

 little during the ages. It is among the more primitive of the Chloro- 

 phyceae or green algae, particularly the fresh-water forms that an- 

 alogies point to the next great step in plant evolution, which was the 

 invasion of the land. This finally resulted in the diversified vegeta- 

 tion of trees, fruits, and flowers as they exist at present. This third 

 stage of plant history, which had its inception in pre-Devonian and 

 probably in pre-Cambrian time, may appropriately be termed the 

 Pteridophytic stage, and from these early pteridophytic stocks have 



