Reconquest of the Water 



these air-spaces has not been explained ; but as the 

 epidermis and the tissues generally in such aquatics 

 are very weak and soft, there is but little resistance to 

 growth and expansion ; perhaps the oxygen formed in 

 the green cells passes into the intercellular spaces in- 

 stead of escaping into the water through the cellulose 

 of the epidermis. 



That, however, is but an unproved suggestion, whereas 

 the spongy nature of aquatics is a fact, and one that is 

 of great importance to their livelihood. 



These aerated tissues are extremely buoyant, which 

 gives them many great advantages. Such a floating 

 plant as the Florida hyacinth is quite unsinkable. Its 

 curious rosettes of fleshy leaves with spongy, gouty- 

 looking leaf-stalks are most ingeniously constructed. If 

 one upsets the rosette in the water, it slowly and 

 solemnly rights itself. It seems to grow very quickly, 

 pushing out long buoyant runners, and twenty to thirty 

 of these leaf-rosettes may belong to one plant. 



It has been proved guilty of many crimes. In the 

 St. John's river, for example, its inordinate multiplica- 

 tion stopped the lumber rafts and steamer traffic (for 

 the paddles were choked by the quantities of weed) ; 

 fishing was brought to a standstill, and even the climate 

 became unhealthy through the masses of decaying 

 vegetation.^ One sees the sponginess of water plants 

 turned to advantage in other ways also. Both fruits 

 and seeds being full of air cavities are easily distributed 

 by the water. Sometimes it is the carpel, or it may be 

 the seed-coat that becomes full of cavities. In Nelum- 

 bium it is the central part or axis of the flower that is 

 able to float. 



The sponginess is clearly, then, a '* fitting reaction " 

 due to water life, and has been made use of for quite 

 different purposes. 



126 



