THE HISTORY OF PLANTS 401 



Whether virus particles are "alive" or not depends on how we define 

 life. They can be crystallized without loss of ability to infect cells; but 

 they also reproduce themselves and occasionally mutate. If we assume 

 that the hypothetical protogenes were like viruses in these respects, we 

 must accept the conclusion that the transition from nonliving to living 

 was a gradual process and that there was no single instant at which life 

 came into existence. Even if protogenes were the source of later life, we 

 are wholly in the dark as how they acquired cytoplasm, how they ceased 

 to depend upon the taking of organic molecules from the environment 

 and passed to the synthesis of food from inorganic materials, and how 

 they came to reproduce sexually. We can only assume that mutations 

 occurred and that natural selection came into play. 



The first cells must have been bits of protoplasm enclosed in semi- 

 permeable membranes, containing genie particles and cytoplasm, but 

 without differentiated nuclei and cytoplasmic structures. They would 

 thus have been comparable to the simplest bacteria, and like some of the 

 latter they were doubtless chemosynthetic. The complex sunlight-chloro- 

 phyll mechanism was almost certainly a subsequent development, and 

 food-capturing (animal) cells may actually have arisen earlier than the 

 green plants. One of the chief differences between the primordial cells 

 and nonliving colloid systems was that, like all subsequent protoplasm, 

 they must have been "open" instead of "closed" chemical systems, in 

 which occurred continuous intake of simple food materials, synthesis of 

 organic compounds by use of energy, and outgo of "waste" materials. 

 A closed system tends toward a static condition of internal balance and 

 fixed size; an open system in proper environment and with enough avail- 

 able energy must grow to a size limit imposed by the surface-volume 

 ratio. If such a system were so constituted that it divided whenever it 

 approached the limiting size, it would show the essential reproductive 

 behavior of a cell. 



Speculations such as these appear logical and reasonable and accord 

 with what is known of the physics and chemistry of protoplasm and with 

 the evidence as to the nature of genes and viruses. Yet they lie at present, 

 and perhaps must always remain, in the realm of untestable hypothesis. 



THALLOPHYTES, THE MOST ANCIENT PLANTS 



The records of the first two-thirds of earth history are contained in two 

 very ancient rock series, in most regions buried under layer on layer of 

 younger rocks. These old rocks are exposed in some places, as around 

 Hudson Bay and Lake Superior and in the bottom of the Grand Canyon. 

 The older of the two series, formed during the Archeozoic era, which 

 endured perhaps 900 million years, consists of highly metamorphosed 

 sediments and enormous amounts of intruded igneous rock. Any fossils 



