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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