LECTURE XVI. 



IT is a highly suggestive fact that all the simplest and most 

 primitive forms of plant life are inhabitants of water and it is only 

 among the more highly developed that we find those which are 

 so constructed as to be able to support themselves on land. Of 

 the examples which we have discussed so far only the two fungi 

 are adapted to sub-aerial conditions. Many of the fungi, however, 

 are aquatic and even those which produce aerial organs have the 

 greater part of their mycelium continually immersed in a liquid 

 or moist substratum, or are embedded in the watery tissues of 

 their hosts, and the aerial organs are formed only in connection 

 with mechanisms to secure the distribution of their spores by 

 means of air currents. It is interesting to note that it is in the 

 fungi which are thus sometimes partially adapted to sub-aerial life 

 that we find peculiar tubular organs developed for the intracellular 

 transmission of the gametes to effect fertilisation, so that the 

 transport necessary to effect fertilisation is independent of aerial 

 conditions. 



The simple body of these aquatic or semi-aquatic plants is not 

 differentiated like that of the higher plants into stem, root and 

 leaves. Being for the most part completely immersed in a fairly 

 uniform medium, the raw materials needed for nutrition may be 

 absorbed all over their surface. Hence no advantage would be 

 obtained in the development on their undifferentiated body or 

 thallus of special areas for the absorption of carbon dioxide and 

 for photosynthesis, or for the absorption of mineral salts. These 

 mors simple and undifferentiated plants thus primarily adapted to 

 an aquatic life are conveniently classed together as Thallophyta, 

 a name descriptive of their simple morphology, which is in har- 

 mony with the physiological uniformity of the medium in which 

 they live. 



Our subject for this lecture Marchantia fiolymorpha is a 

 land plant, but one possessing so many of the characteristics of 

 aquatic vegetation that it seems as it were to illustrate in many 

 ways how a land flora may have originated from aquatic and 

 semi-aquatic plants. 



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