1 " I 

 1 / - 



BOTANY OF THE LIVING PLANT 



I, has only been adopted by a few organisms, the chief of which 

 ertain Algae living in still water, a medium of nearly the same 

 vity as themselves. Thus buoyed up the action of gravity 

 upon them is minimised. 



If • spherical robber balloon be filled with water so as to be turgescent, it 



long as ,t is submerged. But if it be lifted out into the 



an it changes its form according to the support, and the larger it is in proportion 



to the linnn its skin the greater will be the deformation. A very large 



one with ■ weak skin will burst. These simple facts are in accordance with 



■ general principle which rales for similar structures of various size. Their 



strength increases as the square of their dimensions, their weight as the cube 



their dimensions. So long as the structure rests in a still medium, of its 



.u approximate specific gravity, no mechanical difficulty need arise. But 



Tig. io6. 



Part of a transverse section of Canlerpa, showing the thick outer wall, and the 

 r- tii ulate rods of cellulose, which act as ties, and give added rigidity. F. O. B. 

 ( • 50.) 



in a medium of less specific gravity the demand for rigidity rises in a higher 

 ratio than the dimensions of the structure. The result of this applied to 

 plants is that a method of construction which suffices for small organisms, 

 consisting largely of water, and exposed to the air, will not suffice for those 

 of larger dimensions. The mechanical demand on the turgor and strength 

 of the cell-wall in order to maintain form rises in a more rapid ratio than the 

 it may be noted that all large I and- growing plants are septate ; also that 

 land-growing Siphonales are small, and the larger ones are all aquatic. 

 In the of the non-septate cell of large dimensions exposed in the air, 



the wall would have to be of such thickness in order to maintain the form of 

 the organism under the influence of gravity, as to be on the one hand wasteful 

 of material, while on the other it would present a formidable obstacle to 

 physiological transit. Such thickening is seen in the large species of Valonia 

 and ( auUrpa, even though they grow submerged. In both of these genera 

 the cell-wall i iderably thickened ; but in the latter additional firmness 



-•( ured for the otherwise feeble structure by numerous cellulose rods, 

 which stretch across the internal cavity and act as ties (Fig. 106). In other 



