COMPARED WITH THOSE OF ANIMALS. 41 



assume a somewhat stellated aspect. In Juncus effusus 

 the common rush, we have a beautiful example of this 

 kind of tissue, the cell being reduced to a six-rayed star, 



Fig. 9. 

 kASSs 



s 





Stellate cells from the petiole of Sparganium ramosum ; a, a, a, lacunas. 



as shown in Fig. 10. In this manner the large lacunae 

 or air-cells, common in most aquatic plants are formed. 



These air-cells, or lacunae, are designed not only to give 

 buoyancy to the leaves and stems of aquatic plants, but also 

 to prevent their tissues from saturation. The air displaces 

 the water, which is thus excluded from entering the tissues 

 of the plant, otherwise than by the ordinary forces of en- 

 dosmosis and capillarity. The formation of the lacunae is 

 not in this instance the result of a mere mechanical rupture 

 of the tissues in the interior of the plant, on the contrary, 

 they are produced by a regular law of development, im- 

 pressed on certain cells of the tissue, which are thus 

 individualized and set apart for this purpose. 



The stellate vegetable cell is of great importance as 

 illustrating the formation of some of the animal tissues. 

 It is obvious, that if the radiating prolongations of the 

 cells, fig. 10, were to coalesce at the points where they 

 come into contact, so as to throw together the cavities of 



4* 



