510 BOTANY 



The association of Schizophyceae and Algae with the higher plants 

 is by no means unknown, some species having always associated 

 with them a specific form. Among the Liverworts, there are sev- 

 eral e.g. Blasia, Anthoceros which always shelter within their 

 tissues colonies of a species of Nostoc. Among the Ferns, Azolla 

 always is associated with a species of Anabsena, and some Cycads 

 have colonies of Nostoc within their roots. The nature of the 

 symbiosis in these cases has not been critically studied, and its 

 significance is not entirely clear. 



Mycorhiza 



This remarkable form of symbiosis has been referred to in the 

 last chapter, and will not be treated further here. 



Somewhat analogous to the Mycorhizae is the role played by the 

 soil Bacteria, especially those which inhabit the tubercles upon the 

 roots of leguminous plants. The Bacteria penetrate the roots, and 

 the formation of the tubercles is physiologically the same as the 

 deformations of the parts of plants due to the attacks of Fungi, and 

 the Bacteria must be considered as parasites. Nevertheless they 

 enable the infected plants to assimilate the free nitrogen of the 

 atmosphere, and thus more than compensate for any loss of matter 

 due to the attacks of the Bacteria upon their tissues. 



REPRODUCTION 



The character of the reproductive parts of plants is often corre- 

 lated with the nature of the environment, and many remarkable 

 adaptations are connected with this, these being especially devel- 

 oped in the Spermatophytes ; but they are by no means wanting 

 among the lower plants. 



In the Algae the development of locomotive organs (cilia) in the 

 zoospores and spermatozoids is, of course, associated with their 

 aquatic habit, and, in such of the higher plants as develop these, it 

 is only in water that they are functional. So, also, the dehiscence 

 of the sexual organs in the Archegoniates is dependent on the 

 presence of water. 



Where the reproductive cells are non-motile, as in the Ehodo- 

 phycese, they are largely dependent upon chance for their distribu- 

 tion, and the same is true of the spores of most Archegoniates, 

 although in these the roughnesses upon their exterior may, in some 

 cases, make them adhere to animals, and thus be transported. The 

 development of protective walls about resting-spores is usually more 

 or less directly associated with the power to resist desiccation, and 



