270 CHARACTERISTICS OF THE BRYOPHYTA 



owing to the fact that they have not as yet varied and produced 

 tissues that can supply them with an adequate amount of mois- 

 ture and protect them against the cHmatic changes. Owing to 

 their habit of growing together in colonies, they often become 

 conspicuous and cover the bare earth, logs and tree trunks 

 with swards and mats that constitute one of the most attractive 

 features of the forest vegetation, and in northern regions they 

 often form the most characteristic feature of the vegetation 

 over large districts. 



102. General Characteristics of the Bryophyta. — Certain 

 groups of the Bryophyta are characterized by thalloid plant 

 bodies, i. e., without distinction of stem, root or leaf, while many 

 have developed into creeping or erect plants with leaf-like organs. 

 All are distinguished by a marked localization of the growing 

 zone, the elongation of the plant being due to the repeated divi- 

 sions of a single apical cell. The reproduction of the Bryophyta 

 shows such marked departures that we realize that a wide gulf 

 separates even the simplest of them from the Thallophyta. 

 Notable among these departures is the absence of zoospores. 

 Only in a single genus are nonciliated bodies formed and dis- 

 charged from certain cells as in the case of zoospores. In this 

 connection it is worth your time to look back and note the 

 trend of plant life. The lowest forms of the green algae were 

 essentially motile. Then the stationary condition arose and 

 the motile state represented by the zoospores became less and 

 less conspicuous. In the bryophytes we perhaps have in the 

 spores, referred to above, an interesting example of the last 

 trace of the motile phase of plant life. The suppression by the 

 sexuafplant of the spore method of reproduction is due to several 

 causes. They are able to increase their numbers in a vegetative 

 manner. Special branches or ordinary shoots become detached 

 from the parent plant by decay of the older parts or by other 

 means and develop into new plants. Buds, called gemmae, 

 consisting of one or more cells, are also very commonly formed 

 on various parts of the plant and becoming detached grow into 

 new plants (Fig. i86). Possibly also the higher organization of 

 the Bryophyta, which renders them more capable of meeting 



