230 



LECTURE XIV. 



water were conducted entirely, or even only to a small extent through the various 

 tissues of the cortex, the fact would soon be recognised, since the leaves of the tree 

 would droop, and finally dry up. This, however, does not happen : on the contrary, 

 they remain quite fresh, thus proving that in spite of the interruption of the cortex 

 exactly as much water flows to them as they require for their transpiration. The 

 pith in the middle of the stem does not come into consideration at all, since it is dry 

 or already destroyed; and in the cases of thicker stems its mass is much too small lo 

 be taken into account here. But even the usually darker coloured and harder heart- 

 wood of the stem and older branches is not concerned with the conduction of water ; 

 since if a ring of younger wood or alburnum is removed at the same time with the 

 ring of cortex, so that only the ' heart ' remains uninjured, the withering of the crown 

 of leaves betrays the fact that the water-supply has ceased. It is thus, put shortly, 

 in the alburnum that the water of trees ascends. But even within the alburnum a 

 difference prevails, in so farthat the dense autumnal wood of any one annual ring 

 is less capable of conveying water than the large-celled spring wood of the same 

 ring ; and it appears, indeed, that in this way the spring wood of any individual 

 annual ring represents an isolated conducting layer, not in immediate communication 

 with the homonymous layer of an older or younger annual ring '. 



It is demonstrated by means of the above experiments, chiefly for the 

 compact woody body of the Conifers and Dicotyledons, that the ascending current 

 of water moves in the wood only; but no such simple proof is possible, on the other 

 hand, in the case of Monocotyledons and Tree ferns. These, as is known, form 

 no proper woody body. Lignified cells are found, it is true, in the xylem of the 

 individual isolated vascular bundles; but the quantity of this wood of the vascular 

 bundles is so small, that it seems scarcely conceivable how the water necessary for 

 evaporation could be conveyed through the thin lignified strands to the huge leaf- 

 crown of a Date palm, for instance. In addition to this, the connection of the 

 vascular bundles in the palm stem does not favour the assumption that only the 

 xylem of the individual bundle can undertake the conveyance of water. The 

 vascular bundles of the stem of a palm commence below as strands of hair- 

 like fineness, which apply themselves where their diameter is extremely small 

 to the older bundles bending out into the leaves. The difficulty disappears, 

 however, if the sclerenchymatous, thick, lignified vascular bundle-sheaths are claimed 

 at the same time as the principal water-conducting organs of the Palms, Dracaenas, 

 and other Monocotyledons. In their anatomical and histological structure, moreover, 

 as well as in their lignification, these sclerenchymatous strands resemble the more 

 solid parts of true wood ; and in view of their considerable diameter, it seems far 

 more probable that the large quantity of water evaporating in the leaf crown ascends 

 in them. If this assumption, which I hold as more than probable, becomes estab- 

 lished, the sclerenchymatous vascular bundle-sheaths in the stem and leaf-stalks of 

 large Ferns must also be regarded in the same manner. 



1 The proof that the autumnal wood of each annual ring offers great resistance to filtration, as 

 well as the heart-wood, and that the former has probably as little to do with the transpiration-current 

 as the heart-wood, is found in my treatise,- ' Uber die Porositdt des Hohes' (Arb. des bot. Inst, in 

 Wzbg. 1879, B. II, § 3). 



