306 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



embedded organ. The pressures developed by growing plant tissues are con- 

 siderable; the primary root of a bean seedling must be enclosed in a plaster 

 cylinder from i.o to 1.5 cm. in diameter, if the bursting of the cast is to be 

 avoided. 



With the retardation of enlargement that occurs in organs confined in plaster 

 casts, there occurs an acceleration in the development of the internal tissues and 

 structural elements. In a bean root that has been thus embedded for from fif- 

 teen to twenty-seven days, fully developed spiral and pitted tracheae are found 

 at a distance above the root tip of only 1.6 mm., while in a similar root growing 

 normally these vessels do not extend farther than to within from 25 to 35 mm. of 

 the tip. In general, a transverse section from near the tip of such a confined 

 root has the same appearance as a similar section taken from 30 to 50 mm. 

 above the tip of a normal root. 



When enlargement is not completely checked but is merely retarded, then 

 the region of elongation is found to be shorter than in normally growing roots, in 



proportion to the growth-retardation to 

 which the root has been subjected. The 

 normal bean root has a region of elonga- 

 tion about 10 mm. long, while this region 

 may frequently be only 5 or 6 mm., or even 

 no more than 3 mm., long in roots in which 

 growth has been artificially retarded by 

 pressure. 



The experiments just described show 

 that growing plant organs may develop rel- 

 atively very great pressures as they react 

 against obstacles to their growth. Pfeffer 

 carried out a series of experiments to de- 

 termine the magnitudes of the forces thus 

 brought into play. Cubes of plastic clay 

 were prepared, with small, shallow open- 

 ings, into which the tips of growing roots 

 were placed. The roots continued to 

 elongate, in spite of the resistance offered 

 by the clay, and penetrated into the cubes. Then iron replicas of the roots 

 were forced into the same cubes into which the roots had penetrated, and the 

 amount of pressure thus required was determined. From many tests Pfeffer 

 concluded that this pressure was as great as from 100 to 140 g. for the Windsor 

 bean. More precise measurements were made with a spring-dynamometer 

 (Fig. 152). To keep the root from bending as the pressure developed, its upper 

 portion was embedded in a fixed plaster of Paris block (Fig. 152, c). The tip 

 was set in a movable plaster block (d) which was pressed downward as growth 

 of the root occurred, thus transmitting the pressure of the growing organ to the 

 spring (I) of the dynamometer. The following table presents the results of a 

 number of experiments of this kind. In each case are given: the duration of the 



Fig. 152. — Pfeffer*s apparatus for 

 measuring the downward pressures de- 

 veloped by growing roots. (After Pfeffer.) 



