116 



SCIENCE 



[N. S. Vol. XL VI. No. 1179 



the mass of one set of isolated leaves is reduced 

 by cutting out pieces from their center while 

 their isolated sister leaves remain intact the 

 mass of shoots produced by the two sets of 

 sister leaves varies approximately in propor- 

 tion with the mass of the leaves." 



If it is true that the geotropic bending of a 

 horizontally placed stem depends upon the 

 mass of material furnished to the stem by the 

 leaf we should expect that a reduction of the 

 mass of the leaf would correspondingly retard 

 the rate of geotropic bending in the stem. The 

 writer has recently carried out such experi- 

 ments and they corroborate this expectation. 

 If two sets of stems of equal length are sus- 

 pended in an aquarium, each with one leaf 

 attached to its apical end, and if the size of 

 the leaf is reduced in one set by cutting away 

 pieces of the leaf, the geotropic bending takes 

 place the more slowly the smaller the mass of 

 the leaf. It is diiEcult to conceive of a more 

 striking experiment. When the mass of the 

 leaf is reduced to zero, the bending is ex- 

 tremely slow. 



4. These experiments suggest that the 

 growth of the cells of a horizontally placed 

 stem which gives rise to the geotropic bending 

 is accelerated by substances furnished to the 

 stem by an apical leaf; and that these sub- 

 stances might be the same as those which serve 

 for the formation of roots and shoots in the 

 isolated leaf. If this were true, a leaf attached 

 to a piece of stem should form a smaller mass 

 of shoots and roots than its sister leaf entirely 

 detached from the stem, since in the former 

 part of the material available for shoot forma- 

 tion should go into the stem. 



It has been known for some time that a 

 piece of stem inhibits the shoot formation in 

 a leaf of Bryophyllum calycinum, but this in- 

 hibition was attributed by former writers to 

 an influence of roots formed on such a piece 

 of stem. By suitable experiments it can be 

 shown, however, that the inhibition takes place 

 also when no roots are formed on the stem. 



It seemed to the writer that the inhibiting 

 influence of the stem on the shoot production 

 sLoeb, J., Science, 1917, XLV., 436; Bot. Ga- 

 zette, 1917 (in print). 



in the leaf was due, as stated, to the absorp- 

 tion of material from the leaf by the stem 

 which would have served for the growth of 

 roots and shoots in the leaf if the latter had 

 been detached from the stem; and that the 

 material flowing from the leaf into the stem 

 was causing the growth of the cells in the lower 

 side of a horizontally placed stem, thereby giv- 

 ing rise to the geotropic bending of the stem 

 (and incidentally also to the callus formation 

 at the base of the stem). If this were true 

 there should exist a simple quantitative rela- 

 tion between the inhibiting power of the stem 

 upon shoot formation in a leaf and the in- 

 crease in the mass of the stem; namely, the 

 two quantities should be approximately equal. 

 The writer has carried out such experiments 

 in large numbers and found that this relation 

 holds true, namely that a piece of stem at- 

 tached to a leaf increases its weight by ap- 

 proximately the same amount by which the 

 shoot production in the leaf is diminished. 

 For these experiments the foUovnug method 

 was adopted. 



5. A piece from the stem of Bryophyllum, 

 containing one node with its two leaves, is cut 

 out from a plant and the stem split longitudi- 

 nally in the middle between the two leaves, 

 leaving one half of the stem attached to each 

 leaf. The half stem is removed from one leaf 

 and weighed directly. The leaf whose half 

 stem is cut off and the leaf with a half stem 

 still attached to it serve for the experiment. 

 After several weeks the amount of shoots in 

 both leaves is determined by weight and it is 

 found that the leaf without stem had produced 

 a larger mass of shoots than the leaf with a 

 piece of stem attached. The latter is then re- 

 moved from the leaf and weighed. It is in- 

 variably found that it has increased in weight 

 and that this increase approximately equals 

 the diminution in the mass of shoots in the 

 leaf under the influence of the stem. The fol- 

 lowing may serve as an example. 



Three sets of experiments were made simul- 

 taneously on 6, 7 and 7 pairs of sister leaves 

 prepared in the way described above; one leaf 

 was without stem and the other with one haK 

 of the split stem. The three experiments dif- 



