ANGIOSPERM^E, DICOTYLEDONES. 757 



Euphorbiales are distributed in every quarter of the globe. The majority are 

 found in the tropics, and several arboreal species form entire woods in those 

 regions. Some grow in marshy lowlands, whilst others inhabit steppes and the 

 rocky declivities of mountains. Euphorbia capitulata grows on the mountains of 

 the Balkan Peninsula. Euphorbia Austriaca stretches as far as the alpine region 

 of the Eastern Alps. Mercurialis perennis attains in the Alps an elevation cor- 

 responding to the upper limit of the Beech-forests. Several annual species of 

 Euphorbia are encountered as weeds in cultivated ground, as far as the limits of 

 the arctic region. Fossil remains have not been definitely ascertained to exist. 

 The number of extant species hitherto identified is about 4000. 



Alliance XLII. Podostemales. 

 Family: Podostemacece. 



Perennial herbs with creeping roots which are fastened to the substratum. The 

 shoots spring laterally from these roots, and are clothed by small scales arranged in 

 two or three rows; these leaves are either entire or pinnately lobed, and they are 

 sheathed at the base. Not infrequently the shoots are transformed into phylloclades, 

 and sometimes shoots and roots are fused together into a thalloid structure. In 

 these cases the assimilation of carbon is effected by the phylloclades as well as by 

 the green branches of the thalloid tissue clinging to the substratum. The branches 

 in question are ribbon-shaped or filiform, and are submerged. The flowers occur 

 singly at the ends of the shoots, or else are sunk in the margins of the phylloclades 

 in rows, and together form a sort of flat club. They are actinomorphic and zygomor- 

 phic, hermaphrodite, monoecious, and dioecious. The floral-leaves are small, greenish, 

 squamous, free, or connate, and are arranged in a 3-5-partite whorl. When the 

 floral-leaves are suppressed, they are replaced by sheathing involucral leaves. The 

 gynaeceum is composed of 1-3 carpels; the ovary is superior, and either unilocular 

 or else divided by delicate partition- walls into three chambers. The ovules spring 

 from cushions of tissue which project from an axial column in the ovary. The 

 number of stamens varies greatly, the flowers being either monandrous, diandrous, 

 or polyandrous. In the last case the stamens are arranged in several whorls. The 

 anthers dehisce longitudinally. The fruit is a capsule. The seeds are very small, 

 and do not contain any endosperm. The embryo has two thick cotyledons. 



The Podostemacese are found in running water, especially in waterfalls, clinging 

 to rocks, loose stones, and stumps of trees which have been stripped of their bark. 

 Almost all of them inhabit the tropics, the only exception being one species in South 

 Africa and one in North America. No fossil remains have been found. The number 

 of existing species hitherto described amounts to 175. 



