86 THE LIFE OF THE PLANT 



the surface of the root and that of the leaf — adapted to 

 its corresponding twofold environment ; both these 

 surfaces are connected through the intermediate organ, 

 the stem. Such is the general physiological scheme of 

 the plant, a scheme that manifests itself very early, 

 even at the lowest stages of plant life. There are weeds, 

 for instance, which consist of single cells, but which 

 have parts analogous to leaf, root, and stem. Here is a 

 weed (Botrydium), not uncommonly found on flooded 

 plains, consisting of a green, round head and a colour- 

 less branching base, something like a root (fig. 24 to 

 the left), by which it is fixed to the soil. This is only a 

 simple bladder, but in it we notice already two physio- 

 logically differentiated parts. Another more striking 

 example is a weed found in the seas of warm countries 

 (fig. 24). A specimen of this sea- weed (Caulerpa) picked 

 up at Puzzuoli, in the bay of Naples, when straightened 

 out on a sheet of paper, covers an area as large as the 

 palm of the hand, and sometimes even larger. This 

 giant cell, one of the largest in the entire organic world, 

 presents expanded parts remarkably similar to green 

 leaves, stems, and colourless rootlets. The stems 

 spread along the bottom of the sea, the leaf-like laminae 

 grow up vertically, while the rootlets bury themselves 

 in the ooze ; and this is nevertheless one and the same 

 cell, having one continuous cavity.^ 



Such dissimilar organs as the root, leaf, and stem 

 must evidently fulfil absolutely different functions ; 

 and therefore in our subsequent study of the life 

 of the plant, we must study separately the life of 

 these organs. 



There is, however, one more question to be settled : 

 when does the period of germination come to an end, 

 and the independent life of a plant begin ? As a matter 

 of fact the period of germination ends the moment the 



1 Fig. 24 represents two species of Caulerpa — the upper one the 

 common Caulerpa pfolijera, found in the Mediterranean. 



