THE FORMATION OF WOOD IN PLANTS. 421 
Of course the defence for these hypotheses is, that there must be a downward current, 
which must have a course and a cause; and the very natural assumption bas been 
that the course and the cause must be other than those which produce the ascend- 
ing current. Nevertheless there is an alternative supposition, to which the foregoing 
considerations introduce us. It is quite possible for the same vascular system to 
serve as a channel for movement in opposite directions at different times. We have 
among animals well-known cases in which the blood-vessels carry a current first in 
one direction and then, after a brief pause, in the reverse direction. And there 
seems an à priori probability that, lowly organized as they are, plants are more 
likely to have distributing appliances of this imperfeet kind than to have two sets 
of channels for two simultaneous currents. If, led by this suspicion, we inquire 
whether among the forces which unite to produce movements of sap, there are any 
variations or intermissions capable of determining the currents in different directions, 
we quickly discover that there are such, and that the hypothesis of an alternating 
motion of the sap, now centrifugal and now centripetal, through the same vessels, has 
good warrant. What are the several forces at work? First may be set down that 
tendeney existing in every part of a plant to expand into its typical form, and to absorb 
nutritive liquids in doing this. The resulting competition for sap will, other things being 
equal, cause currents towards the most rapidly-growing parts—towards unfolding shoots 
and leaves, but not towards adult leaves. Next we have evaporation, acting more on 
the adult leaves than on those which are in the bud, or but partially developed. This 
evaporation is both regularly and irregularly intermittent. Depending chiefly on the 
action of the sun, it is, in fine weather, greatly checked or wholly arrested every evening; 
and in cloudy weather must be much retarded during the day. Further, every hygro- 
metrie variation, as well as every variation in the movement of the air, must vary the 
evaporation. "This chief action, therefore, which, by continually emptying the ends of 
the capillary tubes, makes upward eurrents possible, is one which intermits every night, 
and every day is strong or feeble as circumstances determine. Then, in the third place, 
we have this rude pumping process above described, going on with greater vigour when 
the wind is violent, and with less vigour when it is gentle—drawing liquid toward 
different parts according to their degrees of oscillation, and from different parts according 
as they can most readily furnish it. And now let us ask what must result under 
changing conditions from these variously conflicting and conspiring forces. When a 
Warm sunshine, causing rapid evaporation, is emptying the vessels of the leaves, the 
osmotic and capillary actions that refill them will be continually aided by the pumping 
‘ction of the swaying petioles, twigs, and branches, provided their oscillations are mode- 
Tate. Under these conditions the current of sap, moving in the direction of least sail 
ance, will set towards the leaves. But what will happen when the sun sets? There 1s now 
o EN crines tes ul noa uai LO atc 
eir general substance, or rather through its interstices, 
of their water, will : h th 
, take up the dye to some distance throug s will become charged while compara- 
say neglecting the vessels. At other times, in the same leaves, the vessel 
ù ely little absorption takes place through the cellular tissue. Even in these exceptional cases, 
through cellular tissue is nothing like as fast as the movement through vessels. 
VOL, XXV., ‘ 
however, the move- 
8 N 
