MECHANICAL CONSTRUCTION OF PLANT-BODY 171 



would be to enlarge the single cell as a non-septate sac. The best of 

 this simple but ineffective method has been made by the Algal family 

 of the Siphonales. But it is only under favourable conditions that it 

 can succeed, for it is a method of construction with obvious limitations. 

 L'nless buoyed up by water it is unsuitable except in the case of small 

 organisms. Some relatively small members of the Siphonales, such 

 as Protosiphon or Vaucheria, live in damp situations exposed to the 

 air. But all the larger forms are submerged, and live usually in still 



r 



T 



Fig. 105. 



Caulerpa prolifera. a, growing apex, b, young thallus-lobes. r = znizoids. 

 Notwithstanding its elaborate form this plant is a single non-septate sac. (t Nat. 

 size.) (After Strasburger.) 



lagoons or pools. In Valonia ventricosa, which is a sea-weed, the 

 form of the sac is simply spherical, or pear-shaped, and may be an 

 inch or more in diameter. But in other genera it is more elaborate 

 in form, and may extend to a foot or so in length. Some mimic 

 curiously the creeping shoots of aerial plants ; for instance Bryopsis 

 and Caulerpa (Fig. 105). Many other of the larger forms, however, 

 grow with numerous branches matted together, giving mutual support 

 (Codium), or even cemented together into a solid mass by deposits 

 of lime (Halimeda). Such structural modifications as these show that 

 the non-septate sac is too weak a method of construction for practical 



