There are two specially obnoxious plants whose seeds may be mixed with agricultural seed, to the dismay of the farmer. We refer to Broomrape and Dodder, both of them unable to earn their own living and depending for their existence on the robbing of other plants. Broomrape usually attaches itself to the roots of Hazel, Poplar or Beech and steals its food therefrom, but its fleshy pink stems and flowers may sometimes be seen in clover fields, then clover is its victim.
More common and more destructive is the little Dodder, a member of the Convolvulus family. Its seeds are very minute and when they germinate they give rise to a seedling not unlike a piece of wire. With one end fixed in the ground, the other waves about till it finds a clover plant round which it twines and not only so but it sends out suckers which microscopic examination shows, penetrate the stem of the clover to rob its food. By pulling a Dodder stem away from the clover we can clearly see a number of holes where the suckers have entered.
Fungus diseases and insects wage constant warfare on the farmer’s belongings. That we may better understand the structure of the disease-causing fungi we are about to examine, let us refresh our memories concerning that very common fungus, known as white mould and mentioned in an earlier chapter. The reason fungi cause damage to other plants, the one invariable reason, is that they, being unable to manufacture food for themselves, steal it from the plants on which they grow. Some of them are parasites and steal their food from living animals or plants; others live upon dead animal or vegetable matter and the white mould is one of the latter fungi.
In most of the fungi which concern us we shall find that there is a mass of minute, thread-like structures forming the main body of the fungus and that, here and there, portions grow erect and bear spores. The spores, it is important to remember, serve the same purpose to the fungus as seeds to the flowering plants. They are blown or carried by insects or other agencies to suitable situations for growth, they germinate and form new fungi. They are smaller and lighter than the most minute and dust-like seeds, so that the slightest breeze scatters them far and wide. Let us compare the white mould with a mushroom; at first sight the two plants appear very dissimilar, in reality they are very similar to one another. The mould forms a thick felt of its threads over the substance on which it grows and mushroom spawn if carefully examined under a microscope will be found to consist of very similar threads, sealed together to form thicker root-like structures. The fungi, however, have no roots and these threads are strictly comparable with those of the mould. Here and there the mould sends a single thread into the air and each of these threads is terminated by a little ball which bursts eventually and sets free its contained spores. The same thing occurs with the mushroom; we have the upright growth, not of a single thread it is true but of a number, welded together to form a fleshy stalk; at the top there is, not a ball containing spores but an umbrella-shaped structure whose under surface is composed of a number of “gills” on which the spores grow at the ends of little stalks. If a piece of the mushroom stem be torn into its separate components and examined under the microscope, its similarity to the more simple fungus is evident. One of the gills also may be carefully cut away and examined; the spores will be seen at the end of small forked stalks.
Having progressed thus far in our study of fungus structure, we may examine a few of those which cause damage in farm and garden. For the most part, the thread-like portion of the fungus grows within the plant attacked and only the spore bearing portions appear on the surface. There is one class of fungus, however, the Mildews in which practically all the structure grows on the surface, only a few small, unbranched suckers penetrate the plant attacked, for the purpose of obtaining nourishment.
Though of great interest to the microscopist, the potato disease is often the cause of serious loss to the farmer. Not only potatoes but also tomatoes are attacked. A potato plant suffering from the disease has irregular yellowish or brownish spots upon its leaves in the summertime. An examination of the lower surface of one of these spotted leaves will reveal a silvery white margin to each spot. This portion should be magnified with a fairly high power and care must be taken not to injure the diseased part of the leaf before it is examined. In cases of serious disease, from nearly every pore on the surface of the leaf fungus threads will be seen to issue. The threads are branched and, at the end of each branch, they have a special kind of spore. They look not unlike miniature leafless trees and they give the typical silvery white appearance to the margins of the diseased spots. When the spores of potato disease germinate, the young fungus threads enter the leaf through a pore and for sometime afterwards there is no sign that the plant is diseased. On this account potato disease and many other fungus diseases are rendered more serious in that the farmer is not and cannot be aware that his crop is attacked till the disease has taken a firm hold. Eventually the potatoes themselves become brown, rotten and breeding grounds for bacteria.
A very common plant disease which makes a good study for the microscope, may be found in quantity upon shepherd’s purse, and as it also attacks cultivated plants of the same family, cabbages, cauliflowers and the like, it is of no little importance. In its early stages, the fungus looks like patches of thick white paint upon the plant and where the fungus grows the plant is invariably contorted. As the fungus matures, the skin of the diseased plant splits and a white powder issues. If some of this powder be highly magnified, it will be found to consist of chains of spores, six or seven in a chain. The spores break off singly and each one may start the disease in another plant.
The microscopist who hunts in garden and farm for fungus diseases, will assuredly meet with some examples of that large class known as “smuts.” They are so called on account of the black powder with which the attacked portions of the plants become filled. The smuts are very important but are not of much interest as objects for the microscope, so we will pass them over for subjects of greater interest if of less importance.
Every farmer knows the familiar and destructive fungus known as “rust of wheat,” it is one of a large class of most interesting plants. The “rusts” are interesting to the microscopist on account of their structure and to the botanist because they cannot, like other fungi, complete their lives upon one plant. They derive their popular name from the fact that they look like patches of rust upon the plants on which they live. Some of the greatest living agricultural botanists have spent many years on producing races of wheat upon which rust fungus will not grow. Wonderful success has rewarded their efforts and conferred immense benefits upon farmers. In spite of this, however, we need not despair that we shall be unable to find a specimen for our microscope, though it is happily an undoubted fact that the disease is not so common now as a few years ago.
