232 



THALLOPHYTES. 



i 



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 accompanied by cell-division, the thallus becoming multicellular, so that from a i 

 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 motility is however limited to the male * swarming ' fertilising 

 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-multiplication ; 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 



