This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
As a rule, shade plants are perennial, whilst the annuals and the biennials are to be found amongst the sun plants. A feature of the woods, dominated by the tree type as they are, is the deciduous character of the vegetation, at least in the cold temperate zone. This is an adaptation to climatic conditions necessitated by the relation of the cold winter period to that of summer. No large tree in this country, except the Pine and Yew, is evergreen. The Holly and the Box are lesser trees which have adopted this habit.
The scrub also consists almost entirely of deciduous shrubs or trees. The hardy ligneous climbers are also deciduous, as the Honeysuckle. The Ivy, however, is an evergreen. A large part of the ground flora is made up of deciduous herbaceous perennials. Unless the woodland plants were as a whole perennial, it is difficult to understand how as annuals they could in the short growing period manage to germinate, and develop stems, leaves, flowers, and fruit; for the light is so feeble compared with that of the open pasture that assimilation would not keep pace with the demands of the plant for rapid growth. Hence also the prevalence of vegetative modes of reproduction.
The case is different in the woods. If one excludes beetles, which are very partial in their choice of plants for pollination, and certain types of Hymenoptera, the group of flower-seeking insects is not so conspicuous in close woods as in the open. True Lepidoptera, especially moths, are frequent in woodlands, but the majority are not bearers of pollen. The Honeysuckle is a familiar example of the dependence of long-tubed plants upon crepuscular moths with a long proboscis, such as the Humming-bird Hawk Moth.
The main feature of woodland plants is the prevalence of wind pollination in the case of the trees. The Grasses are also pollinated by the same agency. Another feature is the occurrence of cleistogamy, as illustrated by the Violet and the Wood-sorrel. A considerable proportion are monoecious plants adapted to self-pollination, whereas the dioecious species are in the majority in open habitats.
In a wood the struggle for existence is so great, owing to the abnormal conditions of light and heat and the density of the vegetation, that plants must necessarily adopt special means of dispersal to a distance. The trees themselves have set the example by being practically all dispersed by the wind. In this, again, they have a pull over the other plants, for being lofty their fruits are more likely to be carried the farthest.
The lower strata of plants are also largely dispersed by the agency of the wind. The Rosebay and other Willow-herbs have cottony appendages, which enable the seeds to travel like parachutes and to settle at a distance. Red Campion, Bluebell, and many other plants possess censer fruits, whilst others are propelled by a catapult or explosive mechanism, as in the case of Wood-sorrel and Wood Spurge.
Animal agency is also largely influential in dispersing seeds. Luscious edible fruits, as those of the Cherry, Rowan, etc, are so scattered. Ivy berries serve the birds in winter. Many fruits have hooks which catch in the coats of animals, as Enchanter's Nightshade, Sanicle, Woodruff, Wood Forget-me-not. The Violet is largely distributed by ants. The small seeds, too, of Grasses and orchids are scattered by aid of the wind.