THALLOPHYTES. 
Others which are both externally and internally more and more complex. In the 
simplest stage the whole vegetative body consists of a single small cell of a 
roundish form, the cell-wall of which is thin and smooth, and within which proto- 
plasm, chlorophyll, and cell-sap are only imperfectly separated. Advancing from 
this, progressive development is first displayed in the perfection of the single cell, 
which increases in size, and often attains dimensions unknown elsewhere in the 
vegetable kingdom, the differentiation being either chiefly in the contents, or in the 
external form, i. e. in branching. The growth of the cells may, on the other hand, 
be acconipanied by cell-division, the thallus becoming multicellular, so that from a 
single cell there arises either a row of cells or a segmented filament, a simple plate 
of cells, or finally a massive tissue growing on all sides. Each of these processes 
further presents a great variety of modifications. 
In the simpler Thallophytes a tendency prevails for a larger or smaller portion 
of their existence to be passed in the condition of motile primordial cells, which 
bear more or less resemblance to the simplest Infusoria, and were in fact until 
recently confounded with them. In some cases cells which are already clothed 
with a cell-wall, or assemblages of such cells, remain for a considerable time in a 
motile condition, swimming freely in the water. But these motile conditions are 
always interrupted by long periods of rest, during which growth and increase in 
size usually take place. In many of the more highly developed Thallophytes 
this power of motihty is however limited to the male ' swarming ' fertihsing 
elements, the antherozoids ; and in many cases is not displayed even here. 
Like the structure of the vegetative body, the mode of reproduction of 
Thallophytes also exhibits great variety, commencing with the simplest kinds, 
and progressing finally to modes of reproduction almost as complicated as those 
which are met with in the highest plants. In the simplest cases reproduction 
appears to be coincident with ordinary cell-multipHcation ; the cell which con- 
stitutes the vegetative body grows and divides, each derivative cell then carrying 
on an independent life and repeating the process. In the more highly developed 
forms the unicellular or multicellular thallus continues to grow for a longer time, and 
becomes differentiated externally and internally, until at length at some one spot 
reproductive cells are produced. In most Thallophytes both kinds of reproduction, 
sexual and non-sexual, occur; and in the higher forms an evident alternation of 
generations is manifested. The reproductive organ which becomes separated from 
the mother-plant is almost always a single cell, which however varies greatly in its 
origin, significance, and capacity for development. The nomenclature which gives to 
all these reproductive cells the name ' Spores ' is a very unfortunate one, obscuring 
an insight into the course of development of the different forms, and rendering dif- 
ficult the comparison of members of one group of Thallophytes with those of another. 
The erroneous theory of so-called * Pleomorphy among Fungi was the result of 
a defective perception of the true nature of the different kinds of reproductive organs 
on which the common name of Spore had been bestowed. What we have now 
to say on this point for the purpose of counteracting the prevalent confusion 
of terms is founded on the view explained in Sect. 29 of Book I, on alternation 
of generations and on the relationship to it of the different reproductive organs. 
I begin by designating as Spores the reproductive cells which are produced in the 
