116 life's beginning on the earth 



plants and perhaps also of some animals. In the plants, the 

 enzyme chlorophyl, the green dye of the leaves, transforms 

 the gaseous carbon dioxide of the air into solid material as 

 we have seen; a part of this solid material is probably sol- 

 uble sugar. Cells form around it in some fashion, perhaps 

 similar to our artificial cell. A cell thus formed must swell 

 through the action of osmotic forces; in this manner it con- 

 tributes to the growth of the plant. 



All these observations demonstrate the analogy of real 

 living cells and the artificial cells made of gelatin. And 

 there is more than a superficial similarity. Even the mode 

 of origin of living cells seems to be related to that of the 

 artificial cells. Just as the gelatin drop immersed in tannic 

 acid solution is able to envelop itself in the thin film of the 

 tender semi-permeable membrane, so also certain material 

 obtained from living plants, when immersed in water, 

 readily surrounds itself with a semi-permeable membrane. 



A suitable material for demonstrating this membrane 

 formation is contained in the delicate root hairs of the water 

 plant, Hydrocharis. If we place these delicate root hairs in 

 a drop of a colored watery solution on a glass slide and press 

 them with a cover glass, the contents flow out , forming oil- 

 like droplets; some of these in turn enclose water droplets. 

 Each droplet surrounds itself with a film which is semi- 

 permeable, as demonstrated by the shrinking of these drops 

 in concentrated salt or sugar solution (Fig. 34). 



In oilier cases, the new formation of a membrane can be 

 seen directly, for instance around the squeezed-out content 

 of the egg cells of such low marine animals as the sea-urchin 

 (Arbacia). In general the material obtained from young 

 and growing cells readily surrounds itself with a film capable 

 of regeneration. Of course these oil drops squeezed from 

 cells cannot be regarded as real cells, but we can form 

 at least- an idea about the manner in which living cells 

 originate. 



