332 



ME TA MORPHOSIS 



which may be compared with the potato in all essential points of structure. 

 Normally the tubers germinating underground form several leafy shoots which 

 root at the base and become independent after the exhaustion of the parent 

 tuber. If the tubers be planted in springtime with only their lower ends in the 

 soil, the tubers themselves form roots while the leafy shoots arising at their upper 

 ends form none. The tuber thus is interpolated between the root and the shoot, 

 and it has therefore to perform not only the function of a storehouse of reserves 

 but also of a conductor of materials absorbed from the soil. It remains alive in 

 this condition the whole summer, grows considerably in thickness, and develops 

 cellular elements which are foreign to its nature but which are characteristic of 

 the normal stem, viz. large vessels, sclerenchyma, and wood parenchyma. The 

 need for elements to carry out the functions of conduction, support, and storage 

 acts here, as in cases of regeneration, as a stimulus which induces the satisfying 

 of this demand. 



We owe to Vochting (1899) also a large number of experiments in which 

 tuberous plants were prevented from forming storage tissue. It is interesting 

 to note that these plants then proceed to deposit their reserves in other organs 

 and that these organs assume in consequence an entirely different structure and 

 function. Under normal conditions all phenomena of this kind are not to be 

 traced to the action of correlation. Two examples may be cited. If offsets of 

 Oxalis crassicaulis laden with reserves be cut off and kept in a moist chamber 



they form normal tubers at their apices, but if all 

 the growing points be removed a tuber is formed 

 all the same either by cellular increase at one or 

 two internodes or by the swelling of the scale- 

 leaves. In this case a com/'Z^ie/y-developed 

 organ has taken on a new function and shape, 

 but such cases are rare. (Compare Winkler, 

 1902, Ber. d. bot. Gesell. 20, 500, where in the 

 case described the factors in the process are 

 sur'^fee'^'^or beelroor^x 5". '"After uukuown). Both structurcs are possibly with- 

 Vochting, 1892. out significance in this plant because it neither 



possesses tubers nor can it form them. 

 Boussingaultia baselloides, which develops normal subterranean stem tubers, 

 is even more plastic. Such tubers may be induced to develop on every foliage- 

 bud if the stem be treated as a cutting, and the axillary-bud be kept in the dark. 

 If a cutting be treated so that its base, free of buds, is placed in the earth a large 

 tuber arises at the basal end of the axis from the callus, which lives throughout 

 the year without being able to produce a growing point. We need not enter into a 

 discussion here as to the varied histological alterations which take place in the 

 stem ; we need only note that roots swell into tubers since the plant has no 

 main axis in which to deposit its reserves. This takes place when leaves are 

 used as cuttings. All plants are probably not so variable, still Vochting's 

 researches have made us acquainted with numerous interesting facts, into 

 which, however, we cannot enter further. 



We have as yet only dealt with correlations which we may study by the 

 simple experimental method of comparison of the results of removing or in- 

 hibiting the functional activity of organs. We may now consider a third 

 method of demonstrating correlations, long known practically, and especially 

 illustrated by Vochting's researches, viz. transplantation or artificial budding. 

 Vochting (1892) has made a thorough study of the simpler forms of trans- 

 plantation. He cut out a cube of beet and then replaced it in the original 

 wound. A rapid healing of the wound took place by appropriate fusions, the 

 cells which were not injured by the cut beginning to swell out, bud (Fig. 99), 

 and grow together where they touched each other (Fig. 100, ///). New 



