202 



MONOSTROMA, ULVA, LAMINARIA, &c. LESS. 



colour and very transparent. One of these is the genus 

 Monostroma, of which M. bullosum is a fresh-water species. 

 Examined microscopically the plant (Fig. 43) is found to 

 consist of a single layer of close-set, green cells, the cell-walls 

 of which are in close approximation, so that the cell-bodies 

 appear as if embedded in a continuous layer of transparent 

 cellulose. Thus Monostroma, like Spirogyra, is only one 

 cell thick (B), but unlike that genus it is not one but many 



vlOJ 



B 



FIG. 43. Monostroma. 



A, surface view of M. bullosum, showing the cells embedded in a 

 common layer of cellulose : many of them are in various stages of 

 division. 



B, vertical section of M. laceratum, showing the arrangement of the 

 cells in a single layer. 



(A after Reinke : B after Cooke. ) 



cells broad. In other words, instead of being a linear it is 

 a superficial aggregate. 



To use a geometrical analogy : a unicellular organism 

 like Hsematococcus may be compared to a point ; a linear 

 aggregate like Penicillium or Spirogyra to a line ; a superficial 

 aggregate like Monostroma to a plane. 



Growth takes place by the binary fission of the cells (A), 

 but here again there is a marked and important difference 

 from Spirogyra. In the latter the plane of division is always 

 at right angles to the long axis of the filament, so that growth 



