102 GROWTH OF PLANTS 



of water plants. The freshly harvested dormant cereals are forced to ger- 

 minate by a few days of prechilling,*^^- ^"-^ or by use of low temperatures; 

 also, breaking the coats forces these seeds to vigorous growth. Other dor- 

 mant grass seeds have been found to respond to coat breaking and to 

 stratification, or to prechilling in germinators. This is true of Setaria 

 macrostachya ^^^ and other grass seeds.^^^ Most dormant grass seeds germi- 

 nate with vigor when the coats are broken or greatly weakened by treat- 

 ment with sulphuric acid of various concentrations, and many respond to 

 prechilling or stratification. A careful study of dormant seeds in this 

 family will no doubt give a big list of seeds with non-dormant embryos 

 that respond to stratification. 



Barton ^ found that low-temperature stratification markedly increased 

 the percentage germination and speed of germination of three of the south- 

 ern pines: loblolly {Pinus Taeda), shortleaf (P. echinata), and slash (P. cari- 

 baea). Longleaf pine (P. palustris) seeds germinated much more promptly 

 than seeds of the other three species but were benefited somewhat by 

 stratification. Later, Barton '' extended the studies to other pines and to 

 several other conifers and found that seeds of many of these were benefited 

 by low- temperature stratification. By examination of Table 11, it will be 

 seen that Abies arizonica, Taxodium distichum, Sequoia gigantea, three 

 species of Thuja, several species of Picea, and many species of Pinus are 

 benefited by low-temperature stratification. Jolmstone and Clare ^^ con- 

 firm Barton's findings for Coulter's pine and show that seeds of the Torrey, 

 Digger, knob-cone, and pinon pine are benefited by stratification. 



Fig. 37 shows that one, two, or three months' stratification of Pinus 

 rigida seeds at 5° C (41° F) gives full germination of practically all good 

 seeds 15 days after planting, whereas the non-stratified seeds string along 

 in their germination, giving only 30 per cent after 50 days. 



Fig. 38 shows the effect of stratification on the germination of loblolly 

 pine (P. Taeda) seeds. One to four months' stratification at 5° C (41° F) 

 gave almost complete germination of all good seeds within 20 days. The 

 germination of untreated seeds had scarcely started after 20 days and 

 attained 40 per cent after 100 days. 



In the spring sowdng of coniferous seeds in nurseries, it is important to 

 have seeds come up promptly and completely so the seedlings \vill have 

 attained some size before the dry, hot days of summer. A month or so 

 of low-temperature stratification just previous to spring sowing will accom- 

 plish this. This discovery is of great importance to nursery practice. 

 Stratification is probably superior to fall planting, which is practiced in 

 some nurseries. Stratified spring-so'wn seeds avoid the hazards of a winter 

 in the soil incurring the danger of germination in mid-winter wdth later 

 freezing and killing of the seedlings. Also the ravages of rodents are 

 avoided. Stratification is also much simpler to apply and much more 

 effective than light, which has been mentioned as a factor in the germina- 

 tion of certain coniferous seeds. 



