80 GROWTH OF PLANTS 



occurred when the O2 tension was lowered to 5 per cent, and considerably 

 less than half of normal germination resulted in O2 concentrations below 

 1 per cent. At O2 tensions of 5 per cent or less, rice seedlings made approxi- 

 mately twice as much growth as wheat seedlings (on the basis of increment 

 in dry weight of embryo in air). Reduction in O2 tension inhibited roots 

 of both seedlings more than shoots, and it had least effect on rice shoots." 

 He attributes the ability of rice seeds and seedlings to grow in very low 

 oxygen tension to a well-developed anaerobic, energy-liberating fermenta- 

 tion system within the seed, which is lacking in wheat; in rice the energy 

 liberation in absence of oxygen was 3^ that in air and in wheat K5. In 

 both grains root growth was more limited by low oxygen pressure than 

 plumule or stem growth. 



Morinaga^'' made a study of the germination of many seeds of land 

 plants under water. He listed 34 species that will not germinate under 

 tap water, 25 species that germinate better on moist filters than under 

 water, 18 that germinate well under water, and 21 that germinate under 

 boiled water sealed with paraffin oil. In none of these was there total 

 absence of oxygen, for the seeds themselves contained some oxygen within 

 the intracellular spaces and even the boiled, paraffin-sealed water permitted 

 some diffusion from air. Among the seeds germinating in boiled water 

 were: timothy, Bermuda grass, Canadian blue grass, lettuce, wormwood, 

 celery, alfalfa, and Petunia. From this work it is evident that oxygen 

 requirements for germination vary greatly even among seeds of land 

 plants. 



Now let us consider seeds that will not germinate in full air pressure 

 because the low permeability of the coats to oxygen reduces the oxygen 

 supply to the embryo below that needed for growth. From all that has 

 been said above, such species of seeds might be thought to be rather rare, 

 and they probably are. It is a different story, however, when the restric- 

 tion offered by coats is further increased by environmental factors, such as 

 submersion in water, in water-logged or packed soil, or in soil rich in 

 carbon dioxide. 



The best understood case of seed coats restricting the oxygen supply to 

 the embryo below the minimum needed for germination is the cocklebur. 

 Farmers had claimed that one of the two seeds in the cocklebur germinates 

 the first season after maturity and the other the second season. Arthur ^ 

 investigated this claim of the farmers and in the main confirmed it, although 

 in some cases both seeds in a bur germinated during the first season and 

 in some burs the second seed did not germinate until the third or a still 

 later season. He found that the two seeds differed in their size, shape, 

 and position in the bur. One was borne higher up in the bur, was smaller 

 and convex on its outer face and concave on the inner face. He termed 

 this the "upper seed." The other was borne lower in the bur, was con- 

 cave on the outer face and convex on the inner face, and was called the 

 "lower seed." The lower seed is the one that germinates in the first season 



