DEVELOPMENT OF PLANTS 243 



remain rather small and inconspicuous, owing to the fact that they 

 have not as yet varied and produced tissues that can supplv them 

 with an adequate amount of moisture and protect them against 

 the climatic 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 feat- 

 ure of the vegetation over large districts. 



102. General Characteristics of the Bryophyta. — Certain 

 groups of the Bryophyta are characterized by thalloid plant 

 bodies, i. c, without distinction of stem, root or leaf, while manv 

 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 rrom the Thallophyta. Nota- 

 ble among these departures is the absence of zoospores. Only 

 in a single genus are nonciliated bodies formed and discharged 

 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 repre- 

 sented by the zoospores became less and less conspicuous. In 

 the bryophytes we perhaps have in the spores, referred to above, 

 an interesting example of tire last trace of the motile phase of 

 plant life. The suppression by the sexual plant 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. 186). 

 Possibly also the higher organization of the Bryophyta, which 

 renders them more capable of meeting conditions that would be 



