Imágenes de páginas
PDF
EPUB

sun.

clearly due to the transpiration of hot water. (Subsequent experiments showed that it is very easy to push this wilting beyond the power of the plant to recover) 4:15 p. m. The plant stands up well. There is no trace of wilt. March 23, 11 a. m. No sign of wilt. Noon. The lowest five leaves show distinct signs of wilt at the tip of the blade. None of the upper leaves show any trace of it. 12:25 p. m. The wilting is worse but is still confined to the lower leaves. It is very decided on the lowest one which is exposed to the bright

The tender apical leaves are turgid, as well as those in the mid part of the stem. 1:20 p. m. The leaf next to the lowest one begins to crisp. 3:00 p. m. Blade of lowest leaf but one is now crisp, and the blades of the other four are drying out at the apex and on the margins and between the larger veins. 4:30 p. m. No change. The bulk of the foliage stands up well, including all of the upper leaves March 25, 1:20 p. m. The lower leaves of this plant are dried out to a greater extent than are those of No. 11, but the major part of the foliage is normal and the tips of both vines are noticeably turgid. The drying out of the parenchyma between the veins is also to be seen in the affected leaves of this vine, the larger veins and a narrow border of the leaf parenchyma remaining a bright green. 5:40 p. m. The vine stands up well. It is three days and four hours since the stem was boiled. March 26, 3:00 p. m.

The vine begins to show symptoms of collapsing. All of the petioles are turgid, but the blade of the lowest leaf is nearly dry, that of the next up is wholly dry; those of the next three above are crisp at the apex

and on the mar gins (one-fifth to one-third the surface); the three next up show a trace of drying on their margins, and in all the rest there is a faint suggestion of loss of turgor. March 27, 2:00 p. m. All of the leaves on this vine are now crisp-dry except three at the top which are flabby. The stem and the petioles are still turgid. March 28, 1:30 p. m. The upper three leaves are still flabby, and all of the petioles are still rigid except the tips of some of the lower ones which begin to droop.

This vine gives results confirmatory of the preceding. For more than three days the plant was able to draw all of the water necessary for its use through 33 cm. of dead stem. Probably if air could be prevented from gradually passing through the shriveled stem into these water carrying vessels and interfering with the normal condition of things the plant might continue to draw its water through a dead stem almost indefinitely

5. THE RESULT OF PARASITIC PLUGGING OF THE VESSELS.

From these experiments and those upon the cucumber wilt, which I have published elsewhere, it follows that the downward path of Bacillus tracheiphilus from the inoculated leaf blade into the stem of the cucumber (for an account of this disease see Centr. f. Bakt. u. Par. Allg. I, No. 9–10, 1895) is exactly that made use of by the ascending water current, just as I stated it to be at the Brooklyn meeting of the A. A. A. S., and the general wilt of the foliage may be explained, first, by a functional disturbance, due to the more or less complete clogging of the lumina of the spiral vessels with countless millions of these bacteria which thrive in the alkaline fluid of the vessels, and, second, by a structural disturbance, due to the breaking down (dissolving) of the walls of these spirals and the flooding out and subsequent growth of the bacteria in the surrounding parenchyma and in the pitted vessels, accompanied, of course, by the more or less free entrance of air into the spirals. It is probable, although not enough examinations have yet been made to render this certain, that no leaf wilts from secondary infection until the water carrying spirals in its petiole have become clogged by the bacillus, i. e., that the wilt of the leaf is not induced by the partial clogging of the vessels farther down in the stem. This is the more likely, first, from the fact that there is always a progressive wilt, leaf after leaf, beginning with the ones nearest the point of infection and moving both ways therefrom, and, second, from the fact that very rarely are all of the pitted vessels filled, so that water lifted up from the roots has always the opportunity to pass around the clog in the spirals by way of the unfilled pitted vessels and to enter the spirals once more farther up. Were this not so, i. e., were pitted vessels filled as readily, as quickly, and as fully as the spirals, we should have not the gradual wilt of leaf after leaf up and down the stem, but the sudden collapse of all the leaves beyond the original point of attack. This is exactly what does happen in watermelon vines attacked by Fusarium niveum, (for a brief account of this parasite see Proc. Am. Asso. Adv. Sci., Vol. 43, 1894, p. 289, and Ibid, Vol. 44, 1895, p. ) where the pitted vessels appear to fill with the fungus as soon, if not sooner, than the spirals.

1 Those who wish to follow these subjects may consult the above mentioned work by Strasburger, pp. 510-936, where many interesting experiments are detailed.

These two diseases of cucurbits are very interesting from a physiological standpoint, and both parasites lend themselves readily to infection experiments, their slightly different behavior being, perhaps, accounted for by the fact that the fungus is strictly ærobic, while the bacillus is facultative anærobic. Whatever be thought of butter or gelatine, it certainly cannot be maintained that the mere presence of these parasites in the lumina of the vessels destroys the carrying capacity of the uninjured walls, and yet they act quite as effectually as gelatine, paraffin, or cocoa butter plugs, causing, when they fill the vessels only incompletely, a flabbiness of the foliage, which is proportionate to the extent of the plugging and to the activity of the transpiration, and which may give place to complete turgor in periods when the transpiration is small (night, early morning, or damp days), and producing, when they completely fill the lumina of the vessels, an entire collapse of the foliage, from which there is no recovery.

In case of the cucumber this collapse takes place as soon as the spiral vessels leading into any petiole are filled by the bacillus.

EDITOR'S TABLE.

-PROFESSORS in the scientific departments of our schools should exercise their influence to prevent the spoliation of nature that is going on at so rapid a rate in our country. We do not especially refer at present to forest fires which involve so much financial loss that our state and general governments are moving in the direction of their prevention. In passing, however, we must refer to the railroad companies as delinquents in this matter, and insist that heavy fines be imposed on them in all cases where fires can be shown to have originated from locomotives. We counted from the car windows of a train not long since, twelve distinct fires burning near the track in the space of a few miles, in a forest covered region not far from Philadelphia, and no one appeared to pay any attention to them.

We wish, however, to refer to the destruction wrought near our cities by the uprooting of plants and the breaking off of branches for purposes of decoration of public and private houses. Within reasonable bounds the vegetable world furnishes material for such decoration, but the practice is carried beyond the rich resources of nature to meet. Our woods are being rapidly stripped of ornamental plants for miles all round our large cities. In many regions the Epigwa repens is completely destroyed, and the blooms of the dogwood and kalmia no longer appear. Lycopodia are uprooted over large tracts, and must now be brought from considerable distances. Some of the ruin is wrought for church decoration, and the girl-graduate is responsible for more of it. Teachers of the natural sciences can teach their hearers that this cannot go on forever. Especially can they point out that botanical classes should not gather arm-loads of orchids of fastidious habits if they do not wish to see the localities destroyed or the species well nigh exterminated.

The authorities in charge of our public parks might, in some places, profitably change their point of view. A park should not consist principally of graded paths lined with stone curbs or walls, separated by tracts of close shorn grass. Shrubberies of nature's planting should remain, and the vines with which nature festoons the forest should not be cut down. No harm is done if there are places where rabbits may hide, and wild birds may nest. Even an owl or two might be permitted to keep down so far as he or she can, the English Sparrow nui

In fact, a park is not necessarily a place from which nature is excluded. The perpetual clearing of undergrowth means also the ultimate destruction of forest, as the natural succession is thus prevented.

sance.

As an offset to this public and private vandalism, we have near our cities a goodly number of citizens who preserve more or less of nature in their private parks. It will be to these to whom we must look to replenish our stock of native shrubs and herbs, if the vandal continues to have full swing elsewhere.

The forty-fifth meeting of the American Association for the Advancement of Science to commence at Buffalo, N. Y., on August 22d, will be characterized by one feature which is deemed by the society an improvement over previous meetings. No excursions will be made during the working hours of the day during the session, only those occupying evening hours being acceptable. At the close of the meeting the field for such diversions will be clear. The geological excursions have been so arranged as not to conflict with the meetings; and the six scientific societies, which meet about the same time, it is hoped will contribute to the importance of the general gathering. It is anticipated that these arrangements will arrest the tendency to dissipation of energy which has been apparent during the last few years. If the habit of many of the embryologists to absent themselves could be overcome, the full force of the Association would be represented. It is expected that a number of evening lectures will present to the public the latest results of research in America.

RECENT LITERATURE.

Surface Colors :-The object of the little book on this subject' by Dr. Walter, of Hamburg is apparently to furnish zoologists, mineralogists, and chemists with an accurate explanation of certain color phenomena which are not as yet universally understood, and which are incompletely treated even in the best text-books on Physics. The keynote of the whole book is given in a single sentence of the introductory chapter. “The intensity of the light reflected from any body may be calculated by Fresnel's ordinary formula for colorless substances, in the case of those rays which are slightly or not at all absorbed by the

1 Die Oberflächen-oder Schillerfarben, von Dr. B. Walter, pp. VIII + 122, Braunschweig, F. Vieweg und Sohn, 1895.

« AnteriorContinuar »