34° INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



time for the development and ripening of the fruit ; indeed in many 

 types the flowers open before the leaves expand, this ' spring flower- 

 ing habit ' allowing freer access of wind and the early insects for 

 pollination. Also often taking advantage of the brief period before 

 the leaves of the dominants or other tall plants expand and cut oflF 

 most of the sunlight, are the small perennial herbs with persistent 

 underground portions that send up flowering shoots and leaves very 

 early in spring, so constituting the ' prevernal aspect '. They 

 flower and fruit rapidly and often die down soon afterwards, as in 

 he case of the Lesser Celandine {Ficaria vernd) and Virginian Spring- 

 beauty {Claytonia virginiana). Somewhat later are the plants of 

 the ' vernal aspect ', such as the Wood-sorrel {Oxalis acetosella) and 

 Yellow Archangel {Galeobdolon luteum), which flower during the 

 bursting of the buds and expansion of the leaves of the overtopping 

 types. Thus not only are the winter and summer aspects of such 

 forests strikingly different, but also often the spring and autumn 

 ones — for in this last season the brilliant galaxy of falling leaves, 

 including the reds of Maples, the yellows of Birches, and the oranges 

 of various others, makes it in the opinion of many enthusiasts the 

 most beautiful of all. 



Although they naturally often intergrade as well as vary com- 

 plicatedly, some five main types of deciduous summer forests may 

 be recognized in various temperate parts of the world, as follows. 



1. Oakwoods of western and central Europe, which tend to be 

 relatively open and light. The dominants are the Pedunculate Oak 

 {Quercus robur) and/or the Sessile Oak (O. petraea = O. sessiliflora), 

 the most important associated trees including Ash [Fraxinus excelsior), 

 Poplars {Popuhis spp.). Birches {Beiula spp.). Elms {Ulmiis spp.), 

 Alder [Alnus ghitinosa), and Wild Cherry {Priimis avium). These 

 change largely according to the nature of the ground. Smaller 

 trees and tall shrubs flourishing in the comparatively light shade 

 include Hazel {Corylus avellana), Holly {Ilex aquifoHum), Hawthorn 

 {Crataegus monogyjia), Field Maple {Acer campestre), Crab Apple 

 {Pyrus malus), Mountain-ash {Sorhus aucuparia), and Yew {Taxiis 

 haccata). In a still lower layer are found a great variety of under- 

 shrubs and coarse herbs and Grasses, while crvptogamic epiphytes 

 may flourish on the bark of the trees. In areas of prevailingly high 

 atmospheric humidity, some vascular plants may grow as more than 

 fortuitous epiphytes. Ivy {Hedera helix) and Honeysuckle {Lonicera 

 periclymenum) are common woody climbers in European oakwoods. 



2. The more varied and luxuriant mixed forests of eastern North 



