88 BOTANY 



sive than this. There are many other types of plants 

 which differ from the land plant we have considered 

 and differ too among themselves in many respects. 

 They live in various situations, they attain to very 

 different dimensions, and they show a great variety in 

 the details of their internal structure. A very large 

 number of plants, some very humble, others very 

 elaborate, in degree of development, live altogether or 

 almost entirely in water. Many others of simple struc- 

 ture occupy moist situations, such as rocks on the banks 

 of streams, damp earth, or trunks of trees near the 

 ground; others again dwell in hot, arid regions where 

 little water can reach them. Nearly all these forms 

 resemble the higher plants in possessing the green 

 colouring matter; but there are others which have lost 

 it, and which live, therefore, on decaying matter or in 

 the bodies of other plants or of animals. 



If we consider the whole mass of vegetation and com- 

 pare the simple forms we find with others of much 

 complexity, we come to see that there is and has been 

 throughout it a continual advance, though a very slow 

 one, in the direction of greater complexity. This leads 

 us to believe that the most highly organised plants we 

 see to-day have been developed from extremely simple 

 ones during the long ages of the past. 



If we try to determine how this has taken place we 

 are able to form some idea of its course by studying the 

 simple plants existing at the present time and the 

 gradual increase in complexity we can find among them. 

 Very probably the different forms show us the different 

 stages through which development has passed. 



The simplest plant we find to-day is a very small 

 structure living in water. It consists of a single piece 

 of protoplasm with its nucleus ; it is clothed by a thin 

 cell wall and contains the green colouring matter. We 

 can suppose without much fear of mistake that the first 



