MECHANICAL CONSTRUCTION OF PLANT-BODY 139 



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

 the Siphoneae. But it is only under favourable conditions that it 

 can succeed, for it is obviously a weak method of construction. Un- 

 less buoyed up by water it is unsuitable except in the case of small 

 organisms. Some relatively small members of the Siphoneae, 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 

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



FIG. 98. 



Caulerpa prolifera. a, growing apex, b, young thallus-lobes. f=zhizoids. 

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

 (J Nat. size.) (After Strasburger.) 



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. 98). Many other of the larger forms, however, 

 grow with numerous branches matted together, giving mutual support 

 (C odium), 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 

 use. It has only been adopted by a few organisms, the chief of which 

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



