200 MONOSTROMA, ULVA, LAMINARIA, ETC. 



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. 44) 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 imbedded 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 



FlG. 44. 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 Haematococcus 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 



