The student must judge for himself what course to follow in each case. While adhering as far as possible to a consistent plan, he must take care that he does not allow his own judgment to be completely over-ridden by a blind obedience to fixed rules, which if pressed too far may defeat their own ends.
PART II. SYSTEMATIC.
CHAPTER VII.
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 words, the thallus is divided up into compartments by transverse septa, but each division contains more than one nucleus. Such coenocytic plants may show well-marked external differentiation of the thallus into members or parts subserving different functions.
A similar wide range is covered by the methods of reproduction among thallophytes.