402 



THE PANDALES 



thickly populate an extensive marsh. The aerial stems live but 

 for a year and are rather weak, but owing to the sheathing bases 

 of the leaves they become sufficiently rigid to support a heavy 

 foliage and withstand the winds. Notice also the extreme light- 

 ness of these organs. This is due to the larger air spaces which 

 also permit a ready interchange of gases from the leaves to all 

 parts of the plant, even throughout the submerged rhizomes. 

 You would naturally expect to find this structure in all aquatics 

 since the roots at least are submerged and all living cells require 

 an interchange of gases (see page 54). The leaves are long 



mT 



FIG. 279. Sectional view of seed of Veltheimia. The embryo consisting 

 of a large cotyledon, c, and laterally placed stem, s, below which is the root, 

 e, endosperm; mi, micropyle;/, funiculus. 



and narrow and point nearly straight up in the air. This pre- 

 vents shading and permits the association of the plants in dense 

 colonies. The leaves are covered with a waxy coating or bloom 

 to prevent the adhesion of water and the plugging of the stomata. 

 This device is often to be seen on the leaves of plants that are 

 subject to heavy dews or rains. Moisture is frequently not 

 evaporated from the leaves until near midday, and if the stomata 

 become filled with water, there can be no interchange of gases 

 for photosynthesis during this time. 



The flowers are of a very primitive type, consisting of naked 



