PART II. SYSTEMATIC. 



CHAPTER YII. 



THALLOPHYTA. 



The divisions of the plant kingdom dealt with in the 

 following chapters of Volume I. are taken in their natural 

 sequence, beginning with the lowest and passing gradually to 

 the highest groups. The list of the classes and families 

 included in Chapters VII. — XI. is given in the table of 

 contents preceding Chapter I. 



Thallophytes are of the simplest type, but they exhibit a 

 very wide range as regards both the structure and differentiation 

 of the vegetative body and the methods of reproduction. In 

 some cases the individual consists of a minute simple cell which 

 multiplies by cell-division; in others the body or thallus is 

 made up of a number of similar units, while .in a great 

 number of forms there is a well-marked physiological division 

 of labour, as expressed both in the external division of the 

 thallus into distinct organs corresponding in function to the root, 

 stem, and leaves of the higher plants, and further in the high 

 degree of histological differentiation of the tissues. In other 

 thallophytes, again, the thallus is a coenocyte either unseptate 

 or incompletely septate ; that is, the individual consists of a 

 single cell differing from a true plant-cell, in the stricter sense 

 of the term, in possessing several nuclei, in other w^ords, the 

 thallus is divided up into compartments by transverse septa, 

 but each division contains more than one nucleus. Such 



