Meristems 69 



of the plant, but it has yielded little knowledge of the meristem as a 

 living and functioning center of morphogenetic activity. 



In recent years, however, an increasing number of workers have used 

 experimental methods to attack the problems of the meristem with 

 techniques like those which have proved so fruitful in the experimental 

 embryology of animals (Wetmore and Wardlaw, 1951). This part of the 

 plant is a perpetually embryonic region and thus offers many advantages 

 for work of this sort. Most shoot meristems are minute, however, and so 

 enfolded by protective structures that experimental work upon them has 

 had to await the development of specialized techniques. Means for direct 

 operative attack on the meristem have now been developed and have 

 begun to yield valuable results. In this work the Snows, Wardlaw, Ball, 

 Wetmore, and their colleagues have been particularly active. The methods 

 of tissue culture have recently added a wealth of information. Biochemi- 

 cal analysis by means of experiments with growth substances, chromato- 

 graphic techniques, and other methods is yielding further knowledge 

 of meristem physiology. This experimental work has powerfully supple- 

 mented earlier descriptive studies. 



Direct operations on the meristem involve procedures of much delicacy 

 and are performed under a lens by tiny scalpels. Pilkington ( 1929 ) seems 

 to have been the first to do such work. She split the meristem of Lupinus 

 down the middle and found that each half regenerated a normal meri- 

 stem so that the original axis was now divided into two branches. Ball 

 ( 1948 ) , also using Lupinus, divided the meristem into four parts 

 longitudinally and each of the four, by regenerative development, was 

 able to produce a normal shoot. Later ( 1952a ) he went still further and 

 split the meristem into six strips. Each of these, unless it was below a 

 minimum size, regenerated a new meristem and shoot, though leaf for- 

 mation was somewhat delayed and vascular tissue was poorly differen- 

 tiated until leaves had developed. 



A major problem here is to find to what extent the meristematic tip is 

 autonomous and thus independent from the tissues below it in develop- 

 ment. In a fern, Dryopteris, Wardlaw (1947 and other papers) isolated 

 the apical meristem from the adjacent leaf primordia by four longi- 

 tudinal incisions (Fig. 4-14). It was thus continuous with the rest of the 

 plant only by the parenchyma of the pith below it, all the vascular tissue 

 having been severed. Despite this isolation, the meristem continued to 

 grow and to produce leaf primordia and leaves. Whatever material en- 

 tered it came through undifferentiated parenchyma. Provascular tissue 

 was developed below the tip but this did not make connection with the 

 vascular bundles in the stem below. 



Ball ( 1948 ) did much the same thing in Lupinus, isolating the central 

 axis of the meristem by four deep incisions. In this axis, however (unlike 



