DEVELOPMENT OF PLANTS 403 



about the nut is finally destroyed by decay, while in the hickories 

 the rind becomes leathery and splits into valves, freeing the nut 

 (Fig. 301, C). 



The order of the nettles, Urticales, also has many points in 

 common with the beech order and contains several of our com- 

 mon trees as the elm (Ulmus, Fig. 302), hackberry (Celtis), mul- 

 berry (Morns), osage orange (To.vylon), numerous tropical 

 forms, as the India rubber trees, banyan tree, etc., as well as a 

 variety of valuable herbaceous plants, as the hop and hemp. 

 These plants very generally contain a milky juice, latex, which 

 in the case of Ficus and Castilloa, two of the India rubber trees, 

 furnishes the raw material of India rubber and in the cow tree 

 of South America yields a saccharine, nutritious milky juice. 

 Tough stereome fibers are characteristic features in many of 

 these plants and furnish the hemp derived from the Cannabis, 

 and in Japan paper is manufactured from the fibers of the paper 

 mulberry. The leaves are often provided with rough hairs which 

 sometimes contain irritating acids, as in the nettles (page 35). 

 The flowers are of a somewhat higher type than in the preceding 

 orders, and associated in a variety of dense inflorescences. They 

 are rarely perfect, though rudimentary (sterile) stamens may 

 be developed with fertile pistils or vice versa, thus giving the 

 appearance of perfect flowers (Fig. 302, B-D). The calyx does 

 not adhere to the ovary as in the Fagales, though the sepals often 

 cohere (Fig. 302, D). The ovary usually matures as a nut, but 

 in many cases a kind of drupe (page 365) is formed, owing to 

 the perianth becoming fleshy and enveloping the nut. The fruit 

 of the mulberry is an aggregation of drupes. In the fig the stem 

 envelops the minute curious flowers and becomes fleshy, forming 

 the edible portion of the fruit. 



141. Orders Showing an Advance over Preceding Forms. 

 The buckwheat order, Polygonales, containing such familiar 

 weeds as the sorrel, dock (Rumex), and knotweed or smartweed 

 (Polygonum), has perfect regular flowers with a distinct calyx. 

 These characters also appear in the allied goosefoot order, Cheno- 

 podiales, with its great array of common weeds, as the goosefoot 

 (Chenop odium) , orache (Atriplex), tumbleweed and pigweed 



