AXGIOSPERMiE, DICOTYLEDONES. 



749 



The Centrospermse are extremely rich in inorganic salts, and in the case of many 

 of the species soda is extracted from the ash obtained by burning the plants. The 

 Piperacese contain aromatic and pungent substances; the Urticaceae secrete enzymes 

 in their stinging- hairs (see vol. i. p. 441). The leaves are lobed in Urticacese and 

 Chenopodiacese, in the rest they 

 are undivided and have entire 

 margins. In several Cheno- 

 podiacese the cauline leaves are 

 squamiform, and assimilation is 

 then effected by the green cortex 

 of the branches, which are trans- 

 formed into i^hylloclades. The 

 Piperacese are distinguished by 

 a peculiar distribution of the 

 bundles in the foliage-leaves. 

 The lateral strands do not branch 

 off fi'om the midrib in the usual 

 way, but are appressed to it and 

 can be traced to the base of the 

 lamina. The Urticacese also, 

 particularly the genus Parie- 

 taria, exhibit a peculiar disposi- 

 tion of the bundles (see vol. i. 

 p. 629). The Chenopodiacese 

 are destitute of stipules, the 

 Paronychiacese have large mem- 

 branous stipules which protect 

 the foliage -leaves, the Poly- 

 gonaceas are distinguished by 

 curious sheathing stipules. In 

 the Caryophyllacese and some 

 Paronychiaceas the floral enve- 

 lopes are differentiated into 

 calyx and corolla; in Nyctagin- 

 acese, Amaranthacese, and most 

 Polygonacese there is a petaloid 

 perianth, whilst in Chenopodi- 

 aceas and Urticacefe there is a sepaloid perianth. Tlie perianth in Nyctagmaceaj 

 resembles a corolla most strongly when the bracts are connate and form a 

 sepaloid envelope or involucre, as is the case, for instance, in the ilarvel of 

 Peru (Mirabilis Jalapa, see fig. 425). The lowest portion of the perianth in 

 Nyctaginaceffi continues to grow after the flower has faded and forms a leathery 

 or woody investment to the fruit (see fig. 425 -). In several Chenopodiacese and 



Fig. 425. — Nyctagiuaceae, Mirabilis Jalapa. 

 I Flowering branch. - Fruit inclosed in the persistent base of the peri- 

 anth. ' Longitudinal section through the same; the true fruit is seen 

 within. (After B.aillon.) 



