200 SPIROGYRA LESS, xix 



Sometimes filaments are found in which the protoplasm of 

 certain cells separates from the wall, and surrounds itself 

 with a thick coat of cellulose forming a body which is quite 

 indistinguishable from a zygote (E). There seems to be 

 some doubt as to whether such cells ever germinate, but they 

 have all the appearance of female cells which for some 

 reason have developed into zygote-like bodies without fertili- 

 zation. Such development from an unfertilized female 

 gamete, although it has not been proved in Spirogyra is 

 known to occur in many cases, and is distinguished as 

 pa rthenogenesis. 



When the zygote is fully developed (F) its cell wall is 

 divided into three layers, the middle one undergoing a 

 peculiar change which renders it waterproof: at the same 

 time the starch in its protoplasm is replaced by oil. In this 

 condition it undergoes a long period of rest, its structure 

 enabling it to offer great resistance to drought, frost, &c. 

 Finally it germinates : the two outer coats are ruptured, and 

 the protoplasm covered by the inner coat protrudes as a 

 club-shaped process (G) which gradually takes on the form 

 of an ordinary Spirogyra filament, dividing as it does so into 

 numerous cells. 



Thus in the present case, as in Penicillium and the 

 mushroom, the multicellular adult organism is originally 

 unicellular. 



The nutrition of Spirogyra is purely holophytic : like 

 Haematococcus and Vaucheria it lives upon the carbon 

 dioxide and mineral salts dissolved in the surrounding 

 water. Like these organisms also it decomposes carbon 

 dioxide and forms starch only under the influence of 

 sunlight. 



