PART I — MORPHOLOGY 



CHAPTER I— THALLOPHYTES 



Introductory. — Thallophytes form the lowest great division of 

 the plant kingdom, the name meaning " thallus plants." A thallus is 

 a plant body in which there is little or no differentiation of vegetative 

 organs. Among the higher plants differentiation results in such dis- 

 tinct vegetative organs as stems and leaves. A thallus body does not 

 distinguish thallophytes absolutely, for some thallophytes have dif- 

 ferentiated vegetative bodies, and thallus bodies are found in other 

 groups of plants. However, the greatest display of thallus bodies is 

 found among thallophytes, and the name is reasonably distinctive. 



As the thallophytes include the lowest plants, the group is especially 

 interesting as representing the living forms nearest to the beginnings of 

 the plant kingdom. Among these plants the beginnings of structures 

 are found that are observed to become modified in various ways in the 

 higher groups. A fundamental conception of the plant kingdom is that 

 it begins with simple forms and advances gradually to more complex 

 forms, until the highest group of plants is reached. To appreciate 

 this evolution of the plant kingdom it is necessary to study plants in 

 this order, beginning with the thallophytes. 



A natural classification of thallophytes, which means a classification 

 based upon relationships, is impossible at present, and any presentation 

 of them must be more or less artificial. Two groups stand out con- 

 spicuously, known as Algae and Fungi ; but there are other groups of 

 thallophytes whose relationships are puzzling. Sometimes the latter 

 groups are distributed among algae and fungi, but this is far from 

 satisfactory. In the following presentation the doubtful groups will 

 be kept separate from the true algae and fungi. 



I. MYXOMYCETES 



General description. — These organisms are commonly known as 

 slime molds or slime fungi. They combine characters of plants and of 

 animals in such a way that opinions differ as to whether they should be 



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