382 



BOTANY 



The tissues in these aquatic forms are very simple, and there are large air- 

 spaces developed, as is always the case in plants having an aquatic habit. 



The simplest flowers are found in the Naiadacese and Lilsea. The flowers 

 here may consist of a single stamen or carpel, in both cases developed as the apex 

 of a shoot, and quite destitute of any proper floral envelopes (Fig. 354). In the 

 more specialized forms, like Butomua and Sagittaria (Fig. 355), a conspicuous 

 perianth is developed, and the carpels and stamens are numerous, but all separate. 

 In Triglochin and Aponogeton the carpels are usually three in number, and in 

 the former more or less completely united into a compound pistil. The most 



II 



FIG. 355. A-F, Sagittaria variabilis. A, young inflorescence with pistillate flower 

 (X i). -B, section of gynoecium, enlarged. C, ripe fruit, enlarged. D, staminate 

 flower (xlj). E, single stamen enlarged. F, leaf (X J). G-I, Elodea Canaden- 

 sis. G, flowering plant (x |). H, flower, enlarged. /, cross-section of ovary. 

 J-L, Vallisneria spiralis. J, male inflorescence, flowers enclosed in the spathe. 

 K, open staminate flower, much enlarged. L, female flower (about natural size) ; 

 gy, lobes of the stigma. 



aberrant forms are found in the Hydrocharitacese, which perhaps should not be 

 included in the Helobiese. These are mostly tropical plants, but two genera, 

 Vallisneria and Elodea, are represented by common species in the Eastern 

 United States. The female flower in these (Fig. 355) has a large inferior ovary, 

 and is borne upon a long pedicel which raises it to the surface when ready for 

 pollination, and in Vallisneria coils up afterward, drawing the young fruit under 

 water, where it completes its growth. 



All the marine Monocotyledons belong to the Helobiese. Zostera and Phyllo- 

 spadix are the principal American genera. 



