272 PART IV. VEGETABLE TAXONOMY. 



that they readily break apart to form distinct organisms. There 

 are those, again, in which the cells are somewhat more inti- 

 mately united to form cell surfaces or flat expansions consisting 

 of but one layer of cells ; and there are still others which by 

 cell-division in three planes, form masses. Among these also 

 many gradations may be observed ; some are cell-masses in 

 which the component cells are nearly alike, and there is little 

 difference between different parts of the organism ; and there are 

 others in which the component cells become developed into 

 tissue-like groups, each differing from the other, but all closely 

 inter-dependent, and where the plant grows into a definite form 

 with a tendency to the development of distinct organs. 



In many members of the Series, the multicellular forms are, 

 as in the higher plants, the product of the division of a single 

 cell ; in others, however, the mature plant-body is an aggregate 

 of cells which were originally distinct but have come together to 

 form a community. 



The great majority of the plants of the Series are at some 

 period in their development endowed with the power of locomo- 

 tion. Among the lowest forms the possession of this power may 

 last during a considerable part of the life of the plant, while in 

 the higher forms it is confined to the spore-period, or in some is 

 wanting altogether. 



Their habits of life are also various ; some are aquatic, others 

 terrestrial ; some are chlorophyll-bearing, flourishing only in the 

 light and assimilating mineral matters, while others are chloro- 

 phylless, indifferent to light, living as saprophytes or as parasites. 



They exhibit great variety also in their modes of reproduction. 

 Among some of the lowest forms no mode is known except that 

 of cell-division ; in some others sexual reproduction takes place 

 in its simplest form, namely, by conjugation ; in still others it 

 takes place by the simplest mode of fertilization, which consists 

 in the production of oospores in oogonia, and, lastly, in the 

 highest forms it takes place by that mode of fertilization which 

 results in the production of a fruiting organ, often quite complex 

 in its character, called a sporocarp. 



The Series is sub-divided into the following Classes, which 

 will be considered briefly in their order : (1) The Myxomycetes, 

 (2) the Schizophyta, (3) the Alga, (4) the Fungi, and (5) the 

 Lichenes. 



