This question has been often asked, and occasionally we have been favored with an answer; each respondent proposing his own theory, and sustaining it by such facts and arguments as appeared to him both reasonable and conclusive. By some writers the plant was supposed to exhaust the soil of all its proper nutriment, in a greater or less time; consequently, when that failed, it became unhealthy, and sooner or later perished, unless that nutriment was restored. By others, the deterioration was not so much charged to the lack of nourishment, as to the elimination of excrementitious matter, the throwing off of a foecal slime, which then became a slow or rapid poison to vegetable life.

Of these two opinions, we give in our adhesion to the latter, because it best agrees with our own observations, and is supported by the greatest amount of testimony. Without insisting upon the infallibility^ of our own views, let us see what may be said in support of the excrementitious theory, or that which refers the unhealthiness of plants, in most cases, to their own rejections.

When first promulged, the opinion that plants discharged this deleterious matter at all, became the subject of a very warm and animated discussion. It provoked a considerable degree of astonishment and disbelief in the minds of cultivators; and not a few eminently practical men, under a conviction of its absurdity and fallacy, attempted to refute it, and to expose its numerous inconsistencies. But time has materially softened the asperities of this once spirited controversy, and, as the prejudices of different individuals have either been subdued or removed, it appears almost needless to bring forward any further arguments in its support than have already appeared, more especially as the most convincing proofs of its correctness are daily before us, both in the field and garden. We consider it a fact perfectly incontrovertible, that plants and vegetables do emit from their roots an excrementitious slime of a quality inimical to their health and existence. Take a naturally good soil, and grow in it successive crops of any one kind of plant. In spite of the application of manure in liberal quantity, that soil will become deteriorated, and unfit for the growth of that particular plant.

How is this fact to be explained? Why will not the plants do as well as they did at first? The soil is prepared, it is well manured, but the result is a failure. Now to our mind, the deterioration results, not so much from the abstraction of nutrition, as from the emission of deleterious substances. But we have palpable evidence of this emission.'* Take a bulb in a* growing state, and place it in a vessel of water, but do not change the water. What is the consequence? In a very few days, the roots will become enveloped with a viscid, slimy substance, the water is thick and ropy, and soon begins to smell unpleasantly. Whence comes this exudation but from the roots 1 and unless it is removed by a fresh supply of water, the health of the bulb will be materially injured, and its death certain. Here then is our theory, and we believe it to be generally admitted and established, therefore a few general remarks may be offered relating to the particular influence of vegetable foeces on certain plants, and thence drawing some practical inferences affecting the cultivation of those plants which are most easily and materially injured thereby.

That the soil is deteriorated much sooner by some plants than by others, must be acknowledged by all who have made this subject a matter of investigation, and the reason why they do thus speedily contaminate the earth is - that their excretions are either more abundant, or more virulent in character. What the cause of this difference is, has not as far as we know, been elucidated by any writer, and it yet demands a satisfactory explanation. It is as likely to be the result of repletion as of anything else, - or to state the case differently - it seems that those plants which are supplied with, and are capable of imbibing a larger quantity of liquid nutriment, must necessarily transfuse a greater portion of that sustenance in the form of excrementitious fluids; and, in corroboration of this . opinion, the fact may be adduced, that all those plants possessing very strong and succulent roots, the soonest cause the greatest amount of damage to the soil. If this is a correct solution of the difficulty, it follows that when a plant is by any means over supplied with moisture, it becomes so completely saturated, that an unusual discharge of refuse matter takes 'place, which, accumulating about the roots, generates unhealthiness; it in fact makes the soil what gardeners term " sour." Very frequently, plants grown in pots are observed to be in a sickly condition, and the reason assigned is, " stagnant water." But the true cause is, we think, to be found in the accumulation of excrementitious discharges, increased by too much watering, and retained by bad or defective drainage.

It frequently happens when plants get into this sad condition, that they are taken from the pot, some of the ball of earth removed, and then repotted in fresh soil. But no benefit is derived from the change; they still remain sickly. The only effectual remedy is, to clear their roots of every particle of soil, wash them in clean water, and then pot them in entire fresh compost. Their health and luxuriance will be speedily restored, and they will again flourish with decided energy and vigor. Again, who has not seen orange trees lose every leaf, because their roots have been seriously injured by superfluous moisture. Take them from the tubs, carefully clean and wash the roots in weak soap-suds, and then replace them in their tubs with a fresh soil. Now put them in a warm moist heat, and soon they will commence growing, to be again clothed with healthy and bright foliage. Nor is this process alone confined to orange trees, for many if not most other plants may be treated in a similar manner, and with alike good results. Those who grow the pansy most successfully, always practice washing of the roots, though they may have no knowledge of the principles which should govern them in resorting to the process.