Tree forms are found in a wide range of plants and their reproductive strategies are substantially the same as shrub or herbaceous plant forms. Many trees are wind pollinated which may be an evolutionary adaptation to take advantage of increased wind speeds high above the ground, particularly in the case of those that produce pollen before the leaves emerge.[64] A vast quantity of pollen is produced because of the low likelihood of any particular grain landing on an appropriate female flower. Wind-pollinated flowers of broad-leaved trees are characterised by a lack of showy parts, no scent and a copious production of pollen, often with separate male and female flowers, or separate male and female trees. The male flowers may be high up in the tree, often in the form of dangling catkins. The female flowers may be lower down the tree. The pollen of pine trees contains air sacs which give it buoyancy and it has been known to travel as far as 800 kilometres (500 mi).[65] Tree pollen can cause allergies. A prime example would be hay fever, which can be caused by pollen. A catkin or ament is a slim, cylindrical flower cluster, with inconspicuous or no petals, usually wind-pollinated (anemophilous) but sometimes insect pollinated (as in Salix). They contain many, usually unisexual flowers, arranged closely along a central stem which is often drooping. They are found in many plant families, including Betulaceae, Fagaceae, Moraceae, and Salicaceae. For some time, they were believed to be a key synapomorphy among the proposed Hamamelididae, but it is now believed that this flower arrangement has arisen independently by convergent evolution on a number of occasions.[citation needed] In many of these plants only the male flowers form catkins, and the female flowers are single (hazel, oak), a cone (alder) or other types (mulberry). In other plants (such as poplar) both male and female flowers are borne in catkins. Catkin-bearing plants include many other trees or shrubs such as birch, willow, hickory, sweet chestnut and sweetfern (Comptonia), and also some herbaceous plants such as nettle. The word catkin is a loanword from the Dutch katteken, meaning "kitten", on account of the resemblance to a kitten's tail.[1] Ament is from the Latin amentum, meaning "thong" or "strap". Pines are trees in the genus Pinus (pron.: /?pa?n?s/),[1] in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species. The modern English name pine derives from Latin pinus which some have traced to the Indo-European base *pit- ‘resin’ (source of English pituitary.[2] In the past (pre-19th century) they were often known as fir, from Old Norse fyrre, by way of Middle English firre. The Old Norse name is still used for pines in some modern north European languages, in Danish, fyr, in Norwegian fura/fure/furu, Swedish, fura/furu, and Fohre in German, but in modern English, fir is now restricted to Fir (Abies) and Douglas-fir (Pseudotsuga).