DEVELOPMENT OF SEAWEEDS 93 



attached to some substratum and no longer passively 

 floated about. So attaching organs were developed; 

 in some of the simple filaments they were only a modi- 

 fied cell at the basal end of the thread ; in the bulky 

 plants the whole of the unbranched end was often 

 specially modified to form a kind of grasping organ 

 capable of growing round stones or into crevices. 



We find such anchorage mechanisms still in the larger 

 seaweeds of our coasts. These organs are not true 

 roots, for they anchor the plant only, and do not absorb 

 nutritive or mineral compounds for its use. 



Such a development of the body of the plant was 

 sufficient for aquatic plants such as seaweeds. Even in 

 the largest of them there is only a very slight specialisa- 

 tion of structure compared with that which is necessary 

 for plants when they come to live on land. We can, 

 however, find in many present day forms three systems 

 of tissues, an external protective and absorbing coat, a 

 conducting system, and between them a system of cells 

 belonging to neither, whose function so far has not been 

 very apparent to us. We see that the division of the 

 body of the plant into members does not go so far as to 

 enable us to recognise what we call stem, leaf, or root. 

 It is in some cases undivided, in others very much 

 branched, but its structure is practically the same 

 throughout. We speak of such a form of plant as a 

 thallus, whether branched or simple, massive or minute. 



CHAPTER XII 



THE DEVELOPMENT OF THE REPRODUCTIVE 

 PROCESSES IN THE ALG^E 



WHILE these changes were occurring in the form and 

 structure of the body of the plant, other developments 

 were taking place in the ways in which new individuals 



