Air Vessels, Or Properly Spiral Vessels, are supposed by some botanists to be the only formation by which air is conveyed into the vegetable system; but air has access to many parts of the plant by means independent of the spiral vessels. Spiral vessels differ from spiral cells (or vermiform bodies) only by dimension, so that there is a constant transition from the latter into the former. Both are quite as frequently filled with sap (in the youngest portions of the plant) as with air (in the full-sized organs). They are first perceptible in the bud. The spiral vessels of the wood are to be distinguished from those of herbaceous plants, both as regards their origin and their function. The latter has not yet been fully explained, owing to the diversity of views entertained by different inquirers. Spiral formation begins when the simple cell membrane ceases to exist. This, as well as all other transitions from one form to another, is accompanied by modifications and changes of the chemical constituents of the vegetable body. In some cases the air in the cavities of the plant contains more oxygen than the atmospheric.