LESSON XX 



MONOSTROMA, ULVA, AND NITELLA 



IT was pointed out in a previous lesson (p. 193) that the 

 highest and most complicated fungi, such as the mushrooms, 

 are found on analysis to be built up of linear aggregates of 

 cells to consist of hyphae so interwoven as to form struc- 

 tures often of considerable size and of definite and regular 

 form. 



This is not the case with the Algae or lower green plants 

 the group to which Vaucheria, Caulerpa, Spirogyra, the 

 diatoms, and, in the view of some authors, Haematococcus 

 and Euglena, belong. These agree with fungi in the fact 

 that the lowest among them (e.g. Zooxanthella) are unicellu- 

 lar, and others (e.g. Spirogyra) simple linear aggregates ; but 

 the higher forms, such as the majority of sea-weeds, have, 

 as it were, gone beyond the fungi in point of structure and 

 attained a distinctly higher stage of morphological differen- 

 tiation. This will be made clear by a study of three typical 

 genera. 



Amongst the immense variety of sea- weeds found in rock- 

 pools between high and low water-marks are several kinds 

 having the form of flat irregular expansions or of bladder- 



