1742 A TEXTBOOK OF THEORETICAL BOTANY 



a pendulous variety is known. Under very favourable conditions 

 it may reach a height of 120 ft. It is a native of all parts of Great 

 Britain. 



5. U. hoUandica. Dutch Elm. This is probably a hybrid between U. 

 glabra and U. nitens. It has a short bole and widespreading 

 branches, the lower limbs often being unusually long. It is a 

 hedgerow plant readily propagating by suckers. 



6 U. stricta. Cornish Elm. It is pyramidal in shape with narrow 

 leaves. It is common in western Cornwall and occasionally 

 found in southern England. According to some authorities this 

 Elm is really a variety of U. glabra and is referred to as U. glabra 

 var. cornubiensis. Another variety, U. stricta var. sarniensis, is the 

 Jersey Elm, which is found in southern England and is frequently 

 planted in the Channel Islands. It is probably not a native. 



7. U. americana. American Elm. It is widely distributed in the 

 United States and Canada and is sometimes planted in this 

 country. 



Many species of the genus Uhntis range widely in temperate regions 

 and in Miocene times were common in Greenland. About 130 species are 

 recognized. 



In recent times the Elm has suffered exceptionally severely from a 

 disease known as the Dutch Elm Disease, the cause of which is still open 

 to question, though fungi certainly are associated with it. The result of an 

 attack is the death of the tree and many fine trees both in this country and 

 in Europe have succumbed to the disease. Elm wood is extensively used in 

 carpentry, particularly for coffins, garden furniture and outdoor implements. 

 It does not rot easily. 



The Urticaceae are mostly herbs or small shrubs. They are a large 

 family of some 480 species, generally tropical in distribution, occurring 

 mainly in the New World and in Asia. In the East Indies species of this 

 family form a notable proportion of the vegetation. In Great Britain there 

 are only two genera, Urtica, with three species — U. dioica (Common Sting- 

 ing Nettle), U. iirens (Small Nettle), and U. piliilifera (Roman Nettle) — and 

 Parietaria with the single species P. diffusa (Wall Pellitory). Both these 

 genera are found chiefly in temperate regions. 



In the Common Nettle (Fig. 1620) the sexes are separated in different 

 plants, while in U. iirens both sexes occur in the same inflorescence. The 

 flowers are arranged in a panicle. Each male flower consists of a four- 

 segmented perianth, with four opposite stamens. The latter are bent 

 downwards and inwards in the bud, but when ripe spring violently upwards, 

 the anthers turning outwards so that the loose pollen is liberated in a tiny 

 cloud. Wind currents carry it to the female flowers. In the female flowers 

 the lateral perianth segments are considerably larger than the antero- 

 posterior ones, the ovary is unilocular and contains a single basal ortho- 

 tropous ovule. The fruit is an akene, and the seed is rich in oily endosperm. 



