DEVELOPMENT OF PLANTS 387 



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

 of the leaves they become sufficiently rigid to support a heavy 

 foHage and withstand the winds. Notice also the extreme Hght- 

 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 49). 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 

 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 

 sporophylls arranged in a compact inflorescence that assumes a 

 spike-like structure at the tip of the stem. The sporophylls are 

 protected until mature by modified sheathing leaves (Fig. 280, ^). 

 Each flower in the upper portion of the spike contains only two 

 or three stamens supported upon a short stalk, which is asso- 

 ciated with hair-like outgrowths (Fig. 280, B), while the lower 

 flowers of the spike bear a single pistil each (Fig. 280, C), which 

 consists of a large flat stigma, style and ovary containing a single 

 ovule and supported upon a hairy stalk or pedicel. Note the 

 association of numerous hairs with both the stamens and pistils. 

 These organs are supposed to be sterile sporophylls so that we 

 have here a very good illustration of a primitive type of flower. 

 The microspores are carried to the stigmas of the pistils by the 

 wind. 



Such anemophilous flowers are characterized by several fea- 

 tures well illustrated in the cat-tail. The microspores must be 

 produced in large numbers since the chance of one reaching the 

 stigma of another plant rapidly decreases as the distance tra- 



