CORRELATION OF FACTORS IN ORGANIC GROWTH. 399 



of the leaf ceases at tlie moment when all the water brought np 

 is claimed by these leaves. Correlative operations then come into 

 play. The leaf parts in process of formation then assnme the 

 shape of bud scales, and all further advance in growth is inter- 

 rupted. Very simple examination will show that this conclusion 

 of the terminal bud is not caused by any deficiency of food. If 

 the growing shoot is brought into a room saturated with moist- 

 ure, the bud is not formed. Evaporation is arrested in such a 

 room ; there is no want of water ; and therefore the mechanism 

 is not unloosed which restricts the further development of the 

 bud. Buds already closed may also be started in a new growth 

 in a room saturated with moisture. 



The closing of the buds in the axils of the leaves is condi- 

 tioned on similar causes. These buds may be easily made to ex- 

 pand if the branch is cut off or stripped of leaves. The advantage 

 to the plant of this property of restoring lost leaves is seen after 

 every instance of devastation by caterpillars. If we cut off a 

 young potato stem just above the ground, the parts growing 

 beneath will at once come up out of the earth. They are the 

 threadlike shoots which would otherwise swell at the ends and 

 form potatoes. The want of incoming nourishment acts as a 

 stimulant to the subterranean growth, and excites processes in 

 them by which their nature is completely changed. Instead of, 

 as before, growing straight down into the soil, the sprout directs 

 its end upward, soon appears above the surface, produces green 

 leaves instead of scales, and puts on the appearance of the ordi- 

 nary leaf}'' shoot. Corresponding in principle with the behavior 

 of this potato plant is that of trees which give their side limbs 

 an upward direction when they have lost their leader. 



The internal and external changes which a fir limb has to un- 

 dergo in order to become a leader are hardly less far-reaching 

 than the transformation of an underground potato shoot into an 

 aerial foliage stem. We perceive that the change of the course of 

 the water and food stuff to the highest limb, which occurs after 

 the removal of the leader, is the correlation force by which the 

 metamorphosis is brought about ; for these causes operate as stim- 

 uli to produce a change in the relations of light and gravity, by 

 which a doubly differentiated organ is converted into one reacting 

 on all sides alike in a word, the whole living mechanism is pro- 

 foundly transformed. When the metamorphosis of the lateral 

 limb into a leader is completed, it can no longer be diverted from 

 its upright position, but returns to it with the same persistency 

 with which it formerly resumed the horizontal position. 



Not less striking are other phenomena of correlation connected 

 with the processes of transpiration, but which can be studied only 

 under the microscope. The moisture which plants give out to the 



