510 LIFE-HISTORIES 



of agriculture and hygiene. Aloreover, a peculiar theoretical 

 interest attaches to thallus-plants from the belief that they 

 include representatives of the forms from which all higher 

 living things have evolved. Since the highest differentiation 

 of the plant-body in thallophytes is for the most part a more 

 or less elaborate lobing, the name lobeworts becomes a 

 significant English equivalent for tlie group. 



In our survey of thallophytic types, several classes were omitted 

 as involving unnecessary complication. Nevertheless, it is hoped 

 that the student has gained some idea of the evolution of thallo- 

 phytes which may be useful to him in further study. As helping 

 further to a general conception of this multifarious division let us 

 briefly review the main lines of development along which we may 

 reasonably suppose the reproductive and the vegetative system of 

 lobeworts to have been evolved. 



With all normal creatures it is as if the controlling purpose of 

 life were the production of well-endowed offspring. Organic evolu- 

 tion seems thus to present a series of attempts to find the best ways 

 of achieving this purpose under all possible conditions of existence. 



In the most primitive organisms, typified by Chroococcus, there 

 is no differentiation into reproductive and vegetative parts, for the 

 entire individual becomes the offspring b}^ fission. Growth in a way 

 prepares for fission, but it may also be regarded as a preliminary 

 part of the reproductive process. Hence we have here what may 

 be called vegetative reproduction in its simplest form. It is a method 

 of propagation admiralily adapted to uniforndy favorable condi- 

 tions, such as these little water-plants enjoy; and, so long as favor- 

 able conditions prevail, vegetative reproduction is the promptest 

 possible way of taking fullest advantage of them. We therefore see 

 it retained with more or less modification by organisms which have 

 developed other methods as well, as for example in desmids. When 

 the offspring remain attached, as in Nostoc, colonies arise, and the 

 colony may propagate vegetatively as if it were an individual by 

 dividing into groups of individuals or hormogonia. If the oifspring 

 produced by fission remain not only attached but in intimate or- 

 ganic union, the result is not a colony of unicellular individuals but 

 a multicellular individual composed of subindividual cells. What 

 before was fission has become ceU-dinsion. Since the growth of 

 multicellular organisms is entirely by cell-division, all growth may 

 be traced back to the preparatory part of reproduction. 



We have seen that there is a great difference in the size of vege- 

 table cells, some plants, such as Mucor, having the body consist of 

 a single cell of relatively enormous size; and there are certain alga>, 

 less familiar, with a unicellular body very much larger than this. 

 Such extraordinary development of a single cell seems not to have 



