106 GROWTH OF PLANTS 



Two-Year Seeds. 



Nurserymen often speak of two-year seeds. These are seeds that in 

 nursery practice as well as in nature do not produce seedlings until the 

 second spring after the seeds ripen. Work at the Institute has led to the 

 grouping of these into three physiological categories. 



(1) Seeds that need a period m the soil at good growing temperatures 

 to permit microorganisms or other factors in the soil to disintegrate the 

 coats, followed by a period at a low temperature to after-ripen the dor- 

 mant embryos. These will be discussed under the heading "Seeds with 

 Resistant Coats and Dormant Embryos." 



(2) Seeds that need a warm-temperature period in the soil to produce 

 a root system, followed by a low-temperature period to after-ripen the 

 dormant epicotyl. These will be described under the heading "Dormant 

 Epicotyls." 



(3) Seeds that require a period of low-temperature stratification to 

 induce root growth, followed by a high-temperature period permitting 

 the root to grow, followed in turn by a low-temperature period to after- 

 ripen the epicotyl, which later requires a higher temperature for good 

 gro^vth. These will be discussed under "Seeds That Require Two Low- 

 temperature Exposures." 



Seeds with Resistant Coats and Dormant Embryos. Seeds of this class 

 differ from those of the group just discussed in that the coats (pericarps or 

 other structures) must be removed or partially disintegrated before the 

 embryos are in a condition to after-ripen in low-temperature moist strati- 

 fication. In nature the resistant coats are partially disintegrated by agents 

 in the soil, especially microorganisms. The latter require good growing 

 temperatures for greatest activity. Hence in nature such seeds require a 

 few months in soil at high temperature for removing the coat resistance, 

 followed by a few months at a low temperature for after-ripening of the 

 embryos, after which they ^vill germinate. The coat resistance will be 

 overcome during the first summer in the soil and embryos will after-ripen 

 durmg the second ^^dnter so that the seeds are ready to grow the second 

 spring. In nursery practice, if such seeds are to be grown out-of-doors 

 without special treatment they should be sown in the sprmg. The coat 

 resistance may be overcome by removing the coats mechanically or by 

 corroding them with such agents as concentrated sulfuric acid. By 

 using one of these methods of removing the coat resistance, seeds can be 

 stratified during the first \vinter and made to grow immediately upon 

 planting the first spring. As to seed or fruit coat, there are two classes of 

 seeds in this group: one in which there is no suture line in the coats, and 

 the whole surface of the coat is decomposed when the coat resistance 

 (Symphoricarpos) is removed, and the other in which there is a dehiscent 

 line at which the coat resistance is removed by decomposition. In the 

 latter, the coats open into equal valves {Crataegus) or a valve comes off 

 with dehiscence (Cotoneaster). 



