“Or as a third expedient, still midway of the part to be bedded in the trench, we make with a sharp instrument an oblique incision from below upward, cutting into the wood as far as the marrow. In this way we are enabled to raise a tongue comprising half the thickness of the shoot, and this tongue is held in its lifted position by a small pebble inserted in the slit. This is what we call a Y-shaped incision, because the raised tongue forms with the rest of the stem an opening like that between the two branches of the letter Y. Through the half that remains intact communication with the mother stem is maintained and the needed share of crude sap is received, while from the cut and upraised half adventitious roots are put forth because the course of the descending sap is arrested there. “In order to bring into contact with the damp soil a greater extent of wounded fiber fit for putting forth adventitious roots, it is customary to split the upraised tongue in two and keep the two parts gaping by interposing a small pebble. This method of double incision is used for trees that offer the greatest resistance to successful layering. “To sum up, all these methods and others derived from them have for their object the fostering of adventitious roots by arresting the course of the descending sap at a certain point beneath the soil.” CHAPTER XXXVI SLIPPING “Propagation by means of a slip or scion cut from the parent tree and so placed that it will develop adventitious roots we may for convenience speak of as slipping. The cut end of the slip is set in the ground in some cool, moist, shady spot where evaporation is slow and the temperature mild. For delicate slips the shelter of a bell-glass is often necessary in order to insure the requisite moisture in the atmosphere and thus prevent the slip from drying up before it has sent down roots to make good its losses. For greater surety, if the slip has many leaves, most of the lower ones are removed in order to reduce the evaporating surface as much as possible without compromising the plant’s vitality, which resides especially in the upper part. But in many cases these precautions are needless; thus, to propagate the grape-vine, willow, and poplar, it suffices merely to thrust the detached scion into the ground.