Imágenes de páginas

young bulbs are cut, the plateau (central short stem of the bulb) is often found to be the only diseased part, something inexplicable if we do not admit that the disease has been transmitted to the bulblet from the mother bulb. Such is the usual form of the disease, but it will be understood that there are numerons modifying circumstances, the disease sometimes beginning lower down on the edge of the leaf, or even underground, or progressing more rapidly, the latter especially when the disease attacks full grown leaves and scapes. The most cases of the first stage of the disease are noticed in the field in May, but cases also occur much earlier in the year.

(4) Pathological Histology.-In spring, in the first stage of the disease, when only the tips of the leaves are attacked, microscopic examination shows the bacterial slime to be present in the intercellular spaces of the shrivelling leaf-parenchyma, but always only in small quantity. From this part of the leaf the bacteria may be traced long distances down the vascular bundles, but have not yet reached the bulbs, the latter being still entirely sound. In autumn, on the contrary, cross sections of the bulbs, if not too badly diseased, show numerous yellow dots in the scales, and on microscopic examination these are found to correspond to the xylem part of the vascular bundles (No. 34, pl. I, figs. 9, 10). The vessels of the latter are seen to be full of a thick, yellow slime, which often partially dissolves them. Here and there, the whole xylem part of the bundle may disappear, the yellow slime taking its place. In this way are formed continuous, tubular cavities, filled with isolated cells of the host plant, remnants of spiral threads, and an innumerable number of bacteria. In this stage of the disease, the sieve tubes are not yet attacked, but these are subsequently destroyed, and frequently, also, the parenchymatic tissues outside of the bundles, the substance which unites the cells being first dissolved. The second spring, a microscopic examination of the yellow striped leaves from diseased bulbs shows a similar occupation of the vessels with the same lesions, but in a reverse order, the bacteria being most abundant and the destruction of tissues greatest in the basal part of the leaf. Here the bacteria dissolve the walls of the vessels and make their way into the surrounding tissues, first isolating and then destroying the parenchymatic cells and finally increasing to such an extent that the epidermis is ruptured and a viscid, yellow ooze escapes. The vessels of the scape are filled in the same way, but the bacterial slime was not observed in the roots. The bacteria may be distinguished in the vessels considerable distances in advance of any external or macroscopic symptoms. The bacterial mucilage is a yellow liquid, thick and viscid. Under a low power of the microscope it shows a granular structure, this being due, as we recognize on higher magnification, to the presence of bacteria. Dr. Wakker describes at some length his method of examination : “ The transverse sections must be made with extreme care, such as is almost unknown in the ordinary study of vegetable anatomy. Not only is the affected tissue so soft that it is impossible, in a fresh state, to cut sections sufficiently thin, but there is also danger that the mucilage will be dragged by the knife into parts of the leaf where originally there was not a trace of it. To overcome these two difficulties good use was made of absolute alcohol. The green color was removed by this method while the yellow of the mucilage persisted, and, in consequence, became much more distinct, so that it was easier to distinguish diseased from healthy tissues.”

(5) Direct Infection Experiments.—This disease was studied before Koch's plate method of isolation had come into general use, and most of the infection experiments were made directly from diseased to healthy plants. In the fall of 1882 bacteria were introduced into a bulb of the double white Anna Maria, and when this bulb was cut the next spring it showed distinct signs of the disease. This experiment was frequently repeated and always with the same result. For example, the whole cut surface of the scales of a bulb from which roots and leaves were cut away in summer was smeared with the bacterial slime, and in 14 days the disease was to be found in the vascular bundles of the youngest scales, and shortly after in those of the older. Slightly diseased bulbs are the best parts from which to obtain the bacteria. In badly diseased plants one runs the danger of finding all sorts of things, even Penicillium, in the decayed mass; and from the leaves, before the disease has reached the bulb, it is impossible to get a sufficient quantity of the slime.

On March 27, 1884, small quantities of yellow slime were taken from some slightly affected bulbs (double red Temple of Apollo) and inserted into wounds made for this purpose in the top of the leaves of different varieties. These were examined daily for signs of disease which first appeared, in most cases, only after a month. Distinct symptoms were apparent but unfavorable circumstances caused the loss of the leaves before the downward stripe had progressed very far. These plants stood in pots in the open air and were watered regularly, but the spring was very dry. Although in this case a month elapsed before external symptoms appeared, it is not to be inferred that so long a time always intervenes. On the other hand it is likely that in natural infections even a longer time may elapse before symptoms appear, since countless numbers of the bacteria are used in artificial infections, while natural infections are probably brought about in most cases by the entrance of a few bacteria which would require more time to produce visible results. On Oct. 27, 1884, the small unfolding leaves of each of a number of sound hyacinth bulbs were wounded with a steel pen and some of the bacterial slime inserted into these punctures. The bulbs were then potted, kept in a place free from frost and examined from time to time. On Jan. 13, 1885, one plant showed the disease very distinctly. Two of the three infected leaves had stripes extending downward from the wound, each about 13 mm. Here, also, a long time intervened between the inoculation and the appearance of external symptoms.

Another series of infection experiments was begun Dec. 28, 1885, and completed in the spring of 1886. These plants were also inoculated with bacterial slime taken directly from dis eased plants. These experiments were made on five plants of as many varieties, grown in carafes. All were kept in a cool place until Feb. 13, when they were transferred to a room regularly warmed. The manner of inoculation and the results obtained are here summarized : La Tour d'Auvergne (double

white variety). Bacteria introduced into a wound in the yellow part of a leaf. Results: Feb. 13. In bloom. Two small streaks extending downward from the wound. Feb. 20. Leaf accidentally broken. It was put immediately after into alcohol and subsequently hardened in absolute alcohol, whereupon microscopic preparations made and stained in the way already described, showed three vascular bundles attacked in varying degrees. Two of them evidently corresponded to the two little stripes on the leaf, while in the third bundle the disease had not made enough progress to be visible on the surface of the green leaf. Norma (single red variety). Bacteria introduced into the green tips of three leaves. Results : Feb. 13. In bloom. 1st leaf, nothing. 2nd leaf, a little spot two millimeters above the wound. 3rd leaf, a similar spot below the wound. Feb. 20. No change. Feb. 27.

No change. March 6. No change. March 20. Second leaf broken ; 3rd leaf show little spots 10 mm. below the wound, plant moved into the open air. March 27. No change. April 3.

No change. Coeur blanc (single white variety). Bacteria introduced into a wound in the yellow part of a leaf. Results: Feb. 13. A stripe extending downward from the wound, 4 mm. Feb. 20. In bloom. No change. Feb. 27. Length of stripe 15 mm. March 6. Length of stripe 22 mm. March 13. Length of stripe 25 mm., and small spots 10 mm. lower. March 20. Length of stripe 27 mm., and small spots 22 mm. lower. Plant put into the open air.

March 27. Length of stripe 27 mm., and small spots 35 mm. lower. April 3. Length of stripe 67 mm. Crown Prince Charles of Sweden (double blue variety). Bacteria put into wounds at the green apex of two leaves. Results : Feb. 13. Nothing. Feb. 20. 1st leaf, a downward stripe of 3} mm. from one of the wounds. 2nd leaf, nothing. Feb. 27. 1st leaf, length of stripe 15 mm., and small spots all around the wound. 2nd leaf, nothing. March 6. In bloom. Length of stripe 17 mm. March 13. Length of stripe 18 mm., and a small spot 4 mm. lower. March 20. Very little change. Plant put under a bell jar on a dish containing water. March 27. Length of stripe 22 mm. Bell jar removed because leaf began to turn yellow. April 3. No change. Anna Maria (double white variety). Fragments of diseased tissue introduced into wounds in the green tips of three leaves. Results : Feb. 13. A downward stripe from one of the wounds, length 10 mm. On the other two leaves nothing. Feb. 20. In bloom. Length of stripe, 17 mm. On the other two leaves nothing. March 6. Length of stripe 35 mm., and small spots 10 mm. lower. March 13. Length of stripe 45 mm. The stripe and bordering tissues have dried up for a distance of 35 mm. March 20. Length of stripe 55 mm., and small spots 15 mm. lower. Diseased part dry for a length of 50 mm. Leaf bent by the drying of one side. Plant put out-doors. March 27. Length of stripe 90 mm. Dry for a length of 55 mm.

April 3. Length of stripe 94 mm.

Measurements were not made after April 3, but subsequently all of the diseased leaves were removed, placed in alcohol, hardened in absolute alcohol, and examined microscopically in the same manner as the leaf already mentioned, and with the same result. On the same date, Dec. 28, 1885, a quantity of bulbs, including the above varieties, were also in. fected and were planted out of doors where they were exposed freely to the air. Up to April 3 there were no signs of disease but a little later symptoms appeared in most of the plants. From these experiments the author draws the following conclusions : (1) the Geelziek or maladie du jaune can be induced artificially, and (2) the results of the infection make their appearance a long time after the operation.

(To be continued.)

« AnteriorContinuar »