140 



BOTANY OF THE LIVING PLANT 



specific gravity as themselves, 

 upon them is minimised. 



Thus buoyed up the action of gravity 



If a spherical rubber balloon be filled with water so as to be turgescent, it 

 keeps its form so long as it is submerged. But if it be lifted out into the 

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

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

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

 a general principle which rules for similar structures of various size. Their 

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

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

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

 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 



Fie. 99. 



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

 reticulate rods of cellulose, which act as ties, and give added rigidity. F. O. B. 

 ( x 50.) 



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 

 size. This is the reason why all large land-growing plants are septate ; also 

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

 In the case 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 activity. Such thickening is seen in the large species of Valonia 

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

 the cell-wall is considerably thickened ; but in the latter additional firmness 

 is secured for the otherwise feeble structure by numerous cellulose rods, 

 which stretch across the internal cavity, and act as ties (Fig. 99). In other 

 large types some accessory means of strengthening has to be adopted, such 

 as matting the branches together as in Codium and Penicillus : or cementing 



