122 



GROWTH OF PLANTS 



There is one other pecuharity that Barton and Schroeder ^* found in 

 certain seeds mentioned in Table 16: the epicotyl could not be after-ripened 

 by a low-temperature exposure until it had lengthened sufficiently to break 

 the epicotyl sheath. This stands m contrast to the situation with the 

 peony and other seeds described in the previous section of this chapter; 



Figure 48. Stages in the development of the seedlings of Convallaria majalis. Left to 

 right: Stage 1, the protrusion of the radicle and hypocotyl; Stage 2, the first evidence 

 of shoot development; Stage 3, further growth of the shoot to break through the coty- 

 ledonary sheath. 



in these cases the epicotyl could be after-ripened as soon as the root started, 

 without the necessity of previous elongation. This has been worked out 

 in detail for Convallaria majalis and Smilacina racemosa seeds. Fig. 48 

 shows three stages in the early development of the seedlings of Convallaria.^'^ 

 It must reach the stage sho^vn in "3" of this figure before a cold period 

 for after-ripening the epicotyl is effective. The epicotyl goes into dor- 

 mancy at this stage of growth and not at earlier stages. This is very 

 similar to the situation with Botrychium lanccolatum and with many of 

 our early spring flowers that grow up every spring from bulbs or tubers. 

 The bud that forms the spring shoot is formed the year before and elongates 

 considerably before it goes into dormancy, no doubt to be after-ripened 

 by the cold of winter ready for early spring growth. 



We have seen for Syynphoricarpos orbiculatus seeds that an intermediate 

 constant temperature, 10° C (50° F), can be chosen that will permit all 

 the after-ripening processes to proceed, including overcoming coat resist- 

 ance, after-ripening of the dormant embryo, and finally growth of the 

 seedling. This temperature is not optimum for any of the processes, and 

 consequently lengthens the time for seedling production, in contrast to 

 using the optimum temperatures for each individual process. No doubt 

 in all the cases described above, where temperature is an important factor 



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