June;, 1930 



EVOLUTION 



Pj 



AGE SliVEN 



wliile that of the animals is superimposed parasitic- 

 ally. But both plants and animals die, their bodies 

 being decomposed into inorganic matter by fungi 

 and bacteria. Fungi are evidently degenerate 

 plants, but what are the bacteria? Are they plants 

 or animals, or perhaps organisms more primitive 

 than either, embodying characters of each? One 

 group, the nitrobacteria, are both structurally and 



f Protop,ajm 





P^^f^^ """""^''''"■'•""^ 



FUN6I 



BACTERIA 



■a£^^/ ^a/rf 



■ ^/l^/-£f.<7ff/? 



NITROBACTERIA*^ 



The Cycle of Life 



physiologically the simplest organisms we know and 

 interrupt the organic cycle in a most suggestive 

 way. They have the remarkable power of drawing 

 free nitrogen from the air and decomposing carbon- 

 dioxide without the aid of sunlight, making them 

 even more independent than the plants. For the 

 plants cannot use free nitrogen, but must get nitro- 

 gen in compounds in order to produce the proteins 

 and protoplasm essential for life. The nitrobacteria 

 are able to fix free nitrogen into these compounds 

 upon which the plants depend. Really the plants 

 are parasitic on these bacteria. Are the nitrobac- 

 teria then the original form of life, the common 

 ancestors of plants and animals? Or must we 

 look to those even simpler organisms, so far in- 

 visible, the filterable viruses recently discovered? 

 We really do not know. 



But our problem here is not the origin of life 

 itself, but that of plants. This is really the problem 

 of what group of organisms first used sunlight for 

 food synthesis. The earliest plants probably lived 

 in the water, for the simplest forms still live there, 

 while the more complex are land forms. Between 

 them are mosses and ferns, amphibious in the sense 

 that they live their lives on land, yet depend on 

 water to carry their male sperm cells during fertil- 

 ization. Turning to the water, we find a most sug- 

 gestive primitive group, the Flagellates, named 

 after the flagella at their head ends, whip-like hairs 

 lashed back and forth in swimming. 



There are two kinds ; those with and those with- 

 out green or other colored chromatophorcs. Those 

 with chromatophorcs can produce their own food 

 like plants, seem to be related to the green algae 



and resemble the free swimming reproductive cells 

 of many low green plants. TJie colorless flagellates, 

 on the other hand, show a close structual likeness 

 to cells of such low animals as the protozoa. The 

 flagellates may therefore represent the primitive 

 type from which have evolved both colorless animal 

 forms and green plants. The green flagellates 

 actually do tend to approach the non-motile condi- 

 tion of t^'pical j^lants, while the colorless forms 

 require greater power of movement to go with their 

 animal method of food getting. 



Without definitely calling them plants, botanists 

 consider the flagellates a sort of substratum for the 

 true plants because they approach them in several 

 ways. (1) They have a structure similar to the 

 lowest algae, being motile, more or less oval, with 

 a solid protoplasmic body, a central nucleus, a 

 specialized head end with one or more whip-hairs, 

 and a contractile vacuole. However, the cell walls 

 are unlike those of both plants and animals, being 

 a dead mucilaginous envelope. Also no starch is 

 found, the chromatophorcs are usually discoid and 

 and the cell walls permit amoeboid movements un- 

 usual in plants. (2) The vegetative division occurs 

 during the motile state and is always longitud- 

 inal, a slow constriction into two halves beginning 

 at the fore end, just as in lower plants. (3) Like 

 the lower algae, the resting cells are formed vegeta- 

 tively by a curling and contraction of the cell con- 

 tents, the walls remaining as transparent shells. 

 There is as yet no sexual reproduction. 



For the present, it seems likely that the Flagell- 

 ates are the primitive stock from which plants or- 

 iginated. Plants may have sprung from some other 

 primitive organisms, or the photosynthetic method 

 may have originated independently in several groups. 





Suggestive likenesses between 

 plants and animals. 



flagellates and simple 



The blue-green algae, for instance, are like bacteria 

 in the splitting nature of their multiplication, and 

 are either immotile or slightly oscillating. They 

 seem, however, to bear little relation to other plants, 

 thougli their color indicates they function as plants. 

 Altogether, the flagellates seem to be an ancestral 

 group from which both animals and plants may have 

 sprung. But at best this is speculation, and more 

 facts are needed for definite knowledge. 



