Posts by Christiansen

Pollen is made up of individual pollen grains, which are very often stuck together so that we see only the mass, not the individual pollen grain. Sometimes the pollen is not sticky, as in the case of pine trees or in the ragweed—a fertile cause of hay fever. In these, and hundreds of other plants, the wind will blow great clouds of pollen through the air.

When we stop to consider that a single, or at most a very few pollen grains are all that are necessary—in fact, are all that can be of real service—the enormous wastage of the male fertilizing substance, in order that mating be secured, gives us some idea of how prodigal is nature in this supreme function.

Calcium is an essential plant food element but its physiological use has not yet been definitely established. It seems to stimulate root-development, and certainly gives vigor and tone to the whole plant. It is commonly believed that calcium is in some way connected with the development of cell-wall material. It has been reported that the stems of grasses and cereal plants become stiffer in the presence of ample calcium, but this may be due to greater turgidity rather than to strengthened cell-walls. Calcium remains in the leaves or stem as the plant ripens, but it is not clear that this has anything to do with the stiffness or weakness of the stem, or straw, of the plant. Experiments with algae have shown that in the absence of calcium salts mitotic cell division takes place, showing that the nucleus functions properly, but the formation of the new transverse cell-wall is retarded. This is the only direct evidence that has been reported that calcium has any connection with cell-wall formation.

Stems above ground, which is the most usual form for them, are of many kinds, all serving the purpose of support to the leaves and flowers, and as a means of carrying sap from the roots or underground stems to the upper part of the plant, and also to carry certain foods to the roots from the leaves, of which more anon. In the case of herbs, like goldenrod or daisy, the stem may be apparently all pith on the inside, with only a thin outer coating of harder substance, not unlike bark, but usually green. If we examine the cut-off trunk of a tree, a quite different structure is apparent. Any lumberman can point out at once “heartwood” and “sapwood”, and his distinctions are just as good as those of the scientist, for he says in these two words as plainly as can be said that heartwood is the oldest and sapwood the youngest. The sapwood is nearer the bark and is honeycombed with passages which serve to carry the sap from the roots to the tree top, while just under the bark is the bright, green, living layer, known as cambium, which is renewed each year. The phloem is the carrier for the food made in the leaves to the roots. It is the successive layers of cambium, year after year, that gives to tree trunks their annual rings. The age of almost all trees can be reckoned exactly by counting these, one representing a year’s growth, and the tree’s rate of growth estimated from the closeness of the rings. Fires or droughts, perhaps long forgotten, here find a lasting record in rings so close together as to be all but invisible. The part nearest the center of the trunk is the heartwood, usually quite lifeless, yet in its maturity furnishing us with lumber. It may be and often is completely decayed, without injuring the flow of sap or the life of the tree for many years.