HELOBIE&. 279 



two ovules. Follicles. Triglocliin has long, fine racemes without 

 bracts or bracteoles ; one ovule in each carpel. The carpels in 

 the two native species are united, but separate when ripe as a 

 schizocarp, loosening from below; they open along the ventral 

 suture or remain closed ; a linear central column remains. 

 The most reduced is Lilcea (1-2 sp. Am.) Protogynous. About 10 species. 

 Temp. Fossils in Tertiary. 



Order 2. Potamogetonacese. The aquatic plants belonging 

 to this order are perennial, living entirely submerged, or with 

 floating leaves, and preferring still water. The leaves are alter 

 nate, in some linear and grass-like, in others there is an elliptical 

 floating blade, supported by a linear submerged petiole. Axillary 

 scales. The fruit is generally a syncarp with nuts or drupes; the 

 embryo is curved, of very various forms. 



Potamogeton (Pond-weed). The rhizome is creeping, sympodial 

 (with two internodes in each shoot-generation) ; the inflorescence is 

 a terminal, many-flowered spike, without floral-leaves ; below it are 

 found 2 foliage-leaves placed nearly at the same height, from whose 

 axils the branching is continued cymosely. The flowers are $ 4- 

 merous, naked, and consist only of 4 stamens, with the connectives, 

 broadly developed at the back of the anthers, resembling a perianth, 

 and of 4 free, sessile carpels. They are common plants in fresh water. 

 The spike, during the flowering, is raised above the water. Wind-pollinated and 

 protogynous. Closely allied is Euppia (Tassel Pond-weed), in salt or brackish 

 water. The spike has only two naked flowers, each consisting of 2 stamens find 

 4 carpels. The stalks of the individual carpels are considerably prolonged. 

 Zannichellia (Horned Pond-weed) is monoecious ; the ? -flower consists of 4 

 (2-9) carpels, with membranous, bell-shaped perianth ; long styles ; the $ -flower 

 has 1 (-2) stamens. Althenia. 



Zostera (Grass-wrack) is an entirely submerged, marine plant with 

 creeping rhizome (with displacement of buds) and strap-shaped 

 leaves. The flowering shoots are sympodia with displacement of 

 the axes (Fig. 281). The inflorescence is a peculiar, flatly-com- 

 pressed spike, on owe side of which the flowers are borne (Fig. 28 L). 

 This inflorescence may be considered, no doubt correctly, to be derived from the 

 symmetrical spike of Potamogeton by strongly dorsiventral development, and by 

 a strong suppression of tbe floral parts taking place simultaneously. Two 

 rows of flowers are developed, but of these one is so pressed into the other that 

 apparently only one is present. Each flower consists of only 1 stamen 

 and 1 carpel situated at the same height (Fig. 281) ; the unilocular 

 ovary encloses 1 pendulous ovule and bears a bifid style. As 

 regards the perianth (?) one leaf may be preset (Z. nana, Fig. 

 281 D). The pollen-grains are filamentous. Pollination takes 



