398 PLANT PHYSIOLOGY 



as cucumbers and tomatoes can be grown in northern latitudes. 

 For successful rooting of the seedlings in the relatively severe 

 conditions of the open field, it is very important to have the root 

 system concentrated in a small volume. For this purpose, the 

 seedlings are not only transplanted, but the roots may be pruned. 



In the practice of agronomy, the conviction is widespread 

 that transplanting not only allows an extension of the length of 

 the vegetative period, but that it accelerates growth and increases 

 the yield, owing to a special stimulation. Based on this con- 

 viction, suggestions have been made to transplant into the field 

 such crops as cereals, and special transplanting machines have 

 been designed for this purpose. 



At one time, this practice was much used in Germany. Trials 

 of this method in practical agriculture did not confirm the hopes 

 of its adherents. A more vigorous development of the indi- 

 vidual plants after transplanting found its explanation in the 

 fact that in applying this method the plants were placed much 

 wdder apart than was the case when the usual method of sowing 

 the seeds in rows was followed. In calculating the yield per unit 

 area, it w^as always found that the transplanted crop did not give 

 a higher yield than drilled seeding and that the great expense for 

 labor and costly machines was not justified. Exact physiological 

 experiments performed by Krassovsky have shown also that 

 transplanting causes a temporary check in growth and an expen- 

 diture of reserve substances for the restoration of the roots lost. 

 Hence, this method is always connected wdth a certain increase 

 in the length of the vegetative period and with a decrease in 

 yield. It is of value only in cases when because of early cold or 

 drought the plants cannot finish their development in time to 

 escape unfavorable conditions and when it is important to begin 

 their cultivation in a small space protected by glass while the 

 weather is still cold. 



92. Artificial Propagation of Plants by Cuttings and Layering. 

 Regeneration and Polarity. Root-forming Hormones. RJiizo- 

 caline. — Because of a high degree of physiological independence 

 that is characteristic of almost all parts of a plant and even of 

 its separate cells, the latter have to a considerable extent the 

 capacity for restoring all the lacking organs and of producing a 

 completely new individual. This capacity has been extensively 

 exploited in agriculture, and particularly in horticulture. Since 



