10 REGENERATION 



rooms of the Institute, on account of the sensitiveness of these 

 plants to certain contaminations of the air. All the experiments 

 were carried out in a greenhouse in which the temperature varied 

 between 20 and 30°C., and where it was never below 20°C. In 

 the greenhouse no illuminating gas was used and no smoking 

 was permitted. 



Whoever wishes to repeat the writer's experiments must 

 realize that the prerequisites of pure air, proper temperature, good 

 illumination, and the protection of the plant against parasites 

 must be fulfilled. 



When leaves of Bryophylbim calycinum are separated from the 

 stem each notch of the leaf may give rise to roots and a little 

 later to shoots (Fig. 14). Such growth occurs only in the notches 

 of the leaf. When the notches are cut out from an isolated leaf 

 no root or shoot formation occurs in the rest of the leaf. As 

 long as the leaf is part of a normal plant, these dormant buds in 

 the notches will not, as a rule, grow out. Only in old plants and 

 old leaves (where there is a suspicion that the flow of sap from 

 leaf to stem is more or less completely blocked) may it happen 

 that a leaf gives rise to roots and shoots while still connected 

 with the stem. 



The experiments were made in large aquaria which were filled 

 to the height of 10 centimeters with water. To strings stretched 

 across the top of the aquarium thin horizontal rods of iron were 

 hooked, and from these the leaves or stems used for regeneration 

 were suspended. To keep the air in the aquarium fairly moist, 

 the top of the aquarium was covered loosely with a glass plate. 



Bryophyllurn has, as stated, 2 leaves in each node, which are of 

 equal age and size, and which may be considered to possess 

 approximately equal masses of chlorophyll and other substances 

 per gram dry weight of leaf. Furthermore, the relative effici- 

 ency of the chlorophyll in the unit of mass of two sister leaves may 

 be considered to be about the same. Hence the roots and shoots 

 produced by equal masses of two sister leaves (detached from 

 the plant) in equal times, under equal illumination and equal 

 temperature, may be used to test the validity of the mass law for 

 regeneration. 



It requires several weeks for a leaf or stem to produce new 

 shoots and roots of sufficient quantity to permit exact weighings. 

 If the time allowed for regeneration is too short, so that the roots 

 and shoots are too small, the error made in cutting off the roots 



