DEVELOPMENT OF PLANTS 



369 



owing to the decay of the older portions, the branches become 

 independent plants. In this way a single plant may spread and 

 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 



nu 



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; f, funiculus. 



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

 ness of these organs. This is due to the large 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 46). The leaves are long 

 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 



25 



