CONCLUDING REMARKS 143 



It has always been noticed that gravity has a strong influence 

 on the formation of organs in plants, and Sachs especially has 

 emphasized this fact. A simple explanation of this influence 

 has been offered in this volume; namely, that it is primarily 

 due to a collection of tissue sap in the lower partsof aleaf orastem 

 placed horizontally. This causes a perhaps slight acceleration 

 of growth which causes secondarily a flow of sap to these more 

 rapidly growing tissues, whereby the growth of tissues on the 

 upper side of the plant organs becomes impossible. 



It is a rather striking fact that in contrast with the wide influ- 

 ence of gravity in the arrangement of organs in plants, similar 

 effects in animals are rare. The writer noticed an influence of 

 gravity on the regeneration of organs in a hydroid, Antennularia 

 antennina,^ but this is a great exception. I have often wondered 

 which difference in the structure of plants and animals might be 

 responsible for this difference in the influence of gravity on the 

 formation of organs. If the view expressed in the pages of 

 this book be correct, that this influence of gravity depends upon 

 the collection of tissue sap in the lowest parts of a stem or a leaf, 

 it would seem to follow that such a collection might be more 

 rare in animals than in plants. A collection of tissue sap which 

 is capable of following gravity is known in animals under the 

 pathological condition of edema, but seems to exist only excep- 

 tionally in normal animals. Where this is the case the condi- 

 tions for an influence of gravity on the arrangement of organs 

 might exist. 



Certain points in the regeneration of Bryophyllum were not 

 touched upon in this httle volume since they are not connected 

 with the mass relation, as e.g. why it is that theanlagen for shoots 

 are found only in definite points of the leaf of Bryophyllum, 

 namely, in the notches; and why in a stem the anlagen for shoot 

 formation exists only in definite spots of the node (in the center 

 of the axil of a leaf) and not in the internode, while anlagen for 

 roots exist throughout certain layers of the cortex of the stem. 



It also still remains to be explained that the new shoots regen- 

 erated have always the hereditary character of Bryophijllum and 

 no other. While these two problems remain unsolved for the 

 present, there is no reason to suppose that they are due to any 

 other than purely physico-chemical agencies. 



1 LoEB, J.: " Unlersuchungen zur physiologischen Morphologic der Tiere." 

 II. Wurzburg, 1891. 



