1897] THE CURVATURE OF ROOTS 315 
shoot, with the general result that each organ will be found 
to respond to a number of forces generally limited to two or 
three, though, of course, instances are not lacking where a great 
number of forms of irritability are found to reside in the 
same organ, as, for example, in tendrils. In such instances, 
however, the excessive number of the forms of irritability has 
been developed to meet special ecological conditions, bearing 
upon both the nutritive and reproductive processes, either 
directly or indirectly. Furthermore, the organs of the shoot 
may acquire also the power of special reactions to internal 
forces or stimuli, such, for example, as the carpotropic move- 
ments. 
In a consideration of the localization and distribution of the 
property of irritability attention is to be called to the fact that 
the conditions concerned in the nutritive processes of the shoot 
show an invariably wide diffusion in space, while varying from 
zero to maximum in time. Carbon dioxide exists everywhere 
in the atmosphere in uniform proportions and bathes every 
Part of the shoot. Sunlight is bounded only by the horizon 
line, and may reach any surface of the shoot in diffuse form. 
The chlorophyll processes may then be carried on by the 
subepidermal tissues in any portion of the shoot, and as a con- 
Sequence a greater proportion of the peripheral protoplasm of 
the shoot has developed an irritability to sunlight, although it 
may not always be manifested by organic or external movement, 
or other response. 
The researches of Rothert have shown that a large part of 
the surface of the leaf of Avena and Phalaris exhibits a helio- 
tropic irritability, and some experiments in my own laboratory, 
by Mr. RE. Squires, demonstrate that the laminae of dicoty- 
ledonous leaves exhibit an equal distribution of sensitiveness 
Over their entire surface, and that the leaflets in a compound 
_ Organ are strictly coordinate and equal with respect to their irri- 
tability (22). Those branches of the shoot that have devel- 
eas a0 Se eae Dees 
oe ‘ped special or ecological adaptations exhibit sinuduemaecae os 
© the irritable surface corresponding to the limited diffusion or 
