244 



LECTURE XIV. 



wood situated lower, until finally this movement, extending backwards, proceeds 

 from the foliage of a land-plant down through the stem, into the young roots which 

 absorb the water out of the earth. 



That this motion of the water molecules depends upon the activity of their 

 evaporation from the leaves is obvious at once, and has already been stated as 

 a fact. On the other hand, however, it is also evident that the rapidity of the 

 flow of the water must depend upon certain properties of the wood walls, 

 which we designate by the word conductivity; if this becomes lessened by any 

 means whatever, the evaporation at the leaves need not on that account be 

 diminished to a like extent. If the latter now continue to transpire vigorously, 

 while the supply is smaller than the loss of water, a deficiency of water must finally 

 result in the leaves, and then even in the younger shoot-axes, and they will 



droop ; and since further arrangements (to 

 be considered later) are met with, by 

 means of which, with a want of water in 

 the organs of transpiration, the transpira- 

 tion itself becomes lessened, it follows that 

 under certain circumstances a diminished 

 conductivity can be inferred from di- 

 minished transpiration. The behaviour of 

 cut-off shoots placed with the lower cut 

 surface in water, gives occasion for these 

 reflections. It might evidently be sup- 

 posed that the cut surface of such a 

 shoot w^ould absorb the water in this 

 form, more easily than when the cut surface 

 is still in connection with the wood of 

 the lower part containing water. This is 

 not the case, however, since various experi- 

 ments show that cut-ofT shoots placed in 

 water gradually transpire less and less, 

 and droop, which is evidently caused 

 only by the imperfect conduction of water. 

 If the shoot has been cut through far 

 from the apex, at a part already strongly lignified, or if the lignification reaches 

 right up into the apex when the winter buds are already formed and all the 

 upper leaves are completely developed, then the phenomenon mentioned appears 

 in a small degree only, and the drooping takes place only after some days ; it 

 occurs after a few hours, however, when a young shoot-axis is cut through, which 

 is not yet, or is but little lignified, and in which the lignification does not yet 

 extend into the apical part— e. g. in the case of the apex of a shoot of the Sun-flower 

 20-30 centimetres long, or one of Arislolochia si'pho 40-50 centimetres long. On 

 fixing such drooping shoots into 'a U-tube, as in the accompanying figure, and 

 pressing the water (w) by means of a column of mercury {q) into the cut end, 

 I found that the shoot after some time again became turgescent and tense : the 

 diminished absorption of water was again increased by the pressure, and the power 



FIG. 201— The U-tube is first filled with water ; thei 

 bored india-rubber stopper i, in which the stem of the pi. 

 fitted, is inserted. The shoot droops, as in a. On po 

 mercury into the other limb, so that g' stands some 8-i 

 above q, the shoot becomes turgid, as in * ; and remair 

 even when the level g subsequently stands higher than g'. 



