CASUARINIFLOKJi. 339 



bracts ; perianth cup-like with oblique edge ; stamens usually 

 numerous ; stigmas often divided. P. tremula (Aspen) has received its 

 name from the tremor of the leaves : cf. " to shake like an aspen leaf." 



POLLINATION. The Poplars are wind-pollinated. The Willows Lave sticky 

 pollen and are pollinated by insects. The catkins of the Willows, especially the 

 <? , are more conspicuous, from the numerous, closely-packed, yellow flowers, 

 rich in honey and pollen. The catkins often appear before the foliage and so are 

 much more easily seen, whilst at this time of the year the number of competing 

 honey-flowers is smaller, and the insect visits consequently more numerous. 

 On many catkins of the Willow the flowers open earliest on the side which is 

 turned towards tlie sun and in descending order, i.e. the upper flowers develop 

 before the lower ones. Hybrids frequently appear. 



There are about 180 species existing in the northern, cold and temperate 

 latitudes. Some in the Polar regions are scarcely more than an inch in height, 

 and have a creeping rhizome (Salix herb acea, polar is, reticulata). Fossil forms 

 are found in the Tertiary and perhaps also in the Upper Cretaceous. 



USES. Principally for ornamental trees, as they grow very quickly and are 

 easily propagated by cuttings, S. babylonica, Weeping Willow; S. purpurea ; 

 Populus alba, Silver Poplar ; P. pyramidalis, Pyramid Poplar a form of 

 P. nigra ; P. monilifera, Canadian Poplar. The wood is very poor and little 

 used ; the branches of many Willows are cultivated for basket-making, etc. The 

 wood of the Aspen is used for matches. The bark contains tannin and, in many 

 Willows, a very bitter extract, Salicin (S. pentandra, fragilis). Salicylic acid 

 (officinal) is obtained from Salix. Balsam is extracted from the buds of many 

 Poplars, especially when the leaves are shooting. 



Family 2. Casuariniflorae. 



Trees with verticillate, scale-like leaves forming sheaths at 

 the nodes. Monoecious. Flowers unisexual, ^-flowers in cat- 

 kins; ? in short spikes. Pollen-tube entering the ovule at the 

 chalaza, and not through the micropyle. Ovary 1-seeded, 

 uniiocular. Carpels uniting into a multiple fruit. Only one 

 order. 



Order. Casuarinaceae. Trees (30 species), from Australia 

 and certain parts of S.E. Asia, with peculiar, equisetum-like 

 appearance. The leaves are verticillate, scale-like and united into 

 sheaths. The internodes are furrowed. Branching verticillate. 

 The unisexual flowers are situated in catkins or short spikes. The 

 ^-flower has a central stamen, surrounded by 2 median, scale- 

 like perianth-leaves and 2 lateral bracteoles. The $ -flower has a 

 1-chambered ovary (2 ascending, orthotropous ovules), no perianth, 

 but 2 large, lateral bracteoles which finally become woody and 

 form two valves, between which the nut-like fruit is situated. 

 The multiple-fruits therefore resemble small cones. Casuarina 

 equisetifolia, cultivated, gives " iron- wood." 



