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.
An outstanding feature of a wood or forest, especially in its natural state, is its dense character. It is for this reason that one resorts to it, for its cool and shady character in summer is at once a pleasing contrast to the open fields where the full blaze of the sun is felt. But the density of a wood has a more particular bearing on the component parts of the woodland flora. In the first place, it is the density of the wood that makes the habit of the tree zone. The close ranks of the tree-trunks themselves cause each to have a particular habit, and regulate the mode of branching above. This is well shown where different degrees of closeness are exhibited, as in natural glades or clearings, or where artificial thinning or coppicing is carried out.
The density of a wood also regulates the extent and character of all the lower strata, e.g. scrub and ground flora. Where a wood is dense the scrub may be absent, or as in case of the trees, attenuated, and growth confined mainly to upward extension. In the case of the ground flora the density of a wood will cause the societies to be large or small proportionally, or even absent in many cases, as in a Beech woodland. Apart from this effect on habit, a dense woodland is far moister, darker, and more liable to fungal pests.
The darkness of a wood has less effect upon habit than upon the character of the lower zones, when the absence of light is due to the density of the tree zone. Since plants depend for the formation of starch very largely upon light, it is obvious that this factor is of very great importance. In a dense wood one may see numerous instances of complete etiolation or bleaching, and partial etiolation or variegation. The vigour of plants is also correspondingly affected in other directions, in the size and extent of their parts, the absence of flowering or successful fruiting. Many trees even may not succeed in flowering or maturing seeds in a dark wood.
The prevalence of fungi, which obtain their carbohydrates ready made, is a feature of woodlands, and their existence in a dark wood is due to their ability to adapt themselves in this way. The kindred groups of phanerogamic saprophytes or parasites, such as Broom-rapes, Toothwort, Bird's-nest Orchis, etc, that live on the roots of trees, is another feature of woods, and their dark character has perhaps been here responsible for the differentiation of such groups.
Woods are the principal agents in condensing the moisture of the atmosphere in the bulk, and for this reason their preservation is an actual necessity. It is only since the greater part of the wooded areas have been disafforested that the climate of this country has become so much drier.
One feature of woodland life that should be noticed is the extraordinary wealth of the lower plants or Cryptogams. These depend upon water for the effective fertilization of the ovum by the spermatozoid, which must meet it in water. Hence the habitat they require must be moist, and a woodland is an ideal type of vegetation for this purpose.
Here, too, is the home of those higher plants that are unable to exist in the open glare of the sun and need moist conditions. Another effect of the moisture of the woods that must not be overlooked is the luxuriance of the vegetative organs.