Betula: A plant genus of the family BETULACEAE. The tree has smooth, resinous, varicolored or white bark, marked by horizontal pores (lenticels), which usually peels horizontally in thin sheets.Alnus: A plant genus of the family BETULACEAE that is distinguished from birch (BETULA) by its usually stalked winter buds and by cones that remain on the branches after the small, winged nutlets are released.Betulaceae: A plant family of the order Fagales, subclass Hamamelidae, class Magnoliopsida. They have simple, serrate, alternate leaves. Male flowers are borne in long, pendulous catkins; the female in shorter, pendulous or erect catkins. The fruit is usually a small nut or a short-winged samara.Corylus: A plant genus of the family BETULACEAE known for the edible nuts.Larix: A plant genus in the family PINACEAE, order Pinales, class Pinopsida, division Coniferophyta.Abies: A plant genus in the family PINACEAE, order Pinales, class Pinopsida, division Coniferophyta. Balm of Gilead is a common name more often referring to POPULUS and sometimes to COMMIPHORA.Trees: Woody, usually tall, perennial higher plants (Angiosperms, Gymnosperms, and some Pterophyta) having usually a main stem and numerous branches.Pollen: The fertilizing element of plants that contains the male GAMETOPHYTES.Fraxinus: A plant genus of the family OLEACEAE. Members contain secoiridoid glucosides.Salix: A plant genus of the family SALICACEAE. Members contain salicin, which yields SALICYLIC ACID.Allergens: Antigen-type substances that produce immediate hypersensitivity (HYPERSENSITIVITY, IMMEDIATE).