Coniferous timbers, such as the pines, exhibit clearly defined annual rings, the autumn growth being darker in colour than the inner ring or spring growth. There are no visible pores on a transverse section of the wood, but pith rays can be discerned under a good glass, very fine and numerous. Hard woods have distinct pores visible to the naked eye. When examined on transverse section, viewed under a microscope, these pores show definite groupings and geometrical arrangements in various timbers, the spring group showing best. Oak and beech afford interesting studies of this feature and of the pith and medullary rays which show as lines radiating from the medulla or pith. This is a general characteristic of many hard woods, and when cut, they constitute the silver grain, or figure, so valued in oak, and apparent in beech, although less conspicuous. Generally, botanical soft woods have no pronounced figure or markings, but there are exceptions, such as the yew, especially the burrs of this tree. For further study of this absorbing subject, the reader is recommended to various standard works on timber and trees given in the Bibliography at the end of the book.
Soft woods, such as the pines, firs, spruces, larches, and cedars, have needle-pointed leaves growing in small bunches or groups. Botani-cally also all coniferous trees are classed as soft woods, the cones being the medium of reproduction.
The leaf of the elm-a hardwood tree-has the edge serrated, and the leaves are arranged five upon a stem. The ash has a more pointed leaf, growing thirteen upon a stem, whilst the lime, alder, sycamore, beech, oak, cherry, etc., have each distinctive leaves, varying in shape and in their growth from a stem or twig. Some are spaced exactly opposite each other on the stem with a terminal leaflet as in the ash, whilst the birch, for example, exhibits an alternate spacing of the leaves on the stem at equal distances one from the other.
The study of flowers and seeds is also a fascinating pursuit. Flowers, male and female, fruit and seeds, ranging from the complicated reproductive process of coniferous trees to the simpler process of germination from seeds contained in an edible fruit, such as the apple, cherry, almond, etc., and the seeds contained in husks and shells, such as the horse-chestnut, acorn, and walnut, are from the instructor's standpoint at least, of especial interest for object lessons, introduced at intervals alternating with object lessons on familiar objects, tools, and processes. A few are suggested at the end of this chapter.
As is shown in the preceding chart, differences of latitude are compensated for by differences of altitude, countries near the equator frequently growing species of trees and other botanical growths at certain altitudes characteristic of northern latitudes. A typical example is the northern pine, which flourishes in Norway, Sweden, and Northern Europe generally, and also on the Sierra Nevada, Spain, the mean temperature for the year being almost identical in both cases. It does not, of course, necessarily follow that exactly similar growths will be found in places far apart with a similar mean yearly temperature, but the number of examples is at least striking.
With regard to differences of location, Mr. Dryer says:"The distance to which any species of plant may extend towards the poles, up a mountain side, or into any relatively cold region, depends upon the length and average temperature of the growing season. The distance to which any species of plant may extend towards the equator, or into any relatively hot region, depends upon the average temperature of a period about six weeks at the hottest time of the year."
Trees adapt themselves to conditions partly by variations of leaf surfaces; thus"deciduous" or leaf-shedding trees, including the beech, oak, plane, and sycamore, have broad horizontal leaves with a thin epidermis or skin which permits the maximum working power, viz. sustaining and nourishing the tree-whilst the growing season lasts, by exposure to the air.
Pines, on the other hand, with their spiky, needle-pointed leaves are peculiarly adapted to resist drought, by virtue of the small aggregate leaf surface exposed. In tropical countries atmospheric evaporation is a considerable factor; the majority of the growths are evergreen, and new leaves appear before the old ones are shed in order to sustain the supply of nourishment to the tree.