61 



character and age of the fertiUzing pollen; the nutrition of the parent 

 plant, accidents of sun and shade, moisture and dryness of soil, high and 

 low altitude, weather conditions during ripening, the time of harvesting 

 and subsequent handling of seeds, are influential factors. 



Various other investigators attribute delayed germination in seeds, 

 tested by them, to characters of the embryo, but they leave further 

 details about the essential point out of account. 



We may mention Nobbe and Haenlein (22,23), Kienitz (24), Wiesner 

 (25), Lakon (26), Piichner (27), Crocker and Harrington (28, 29). 



The conception that delayed germination is a resiilt of seed coat 

 characters has also many advocates. The following cases are dis- 

 tinguished by them : — 



(1) Seeds in which delayed germination is due to the imper- 

 meability of the seed coat to water. Nobbe and Haenlein (22) 

 assume that the cause of the resistance of clover seeds to water 

 must be considered to originate in the outer cell layer, the palisade 

 layer. Hiltner (30) suggests: " Nicht der hohe Wassergehialt der 

 Nachreife bediirftige Getreide, sondern im Gegenteil deren Unf ahig- 

 keit das zum Auslosung des Keimungsactes notwendiges Wasser in 

 aich aufzunehmen, bedingt ihre Tragheit in der Keimung." 



(2) Seeds in which the non- germination is simply due to the 

 fact that only a subminimal quantity of oxygen can reach the 

 embryo. Crocker (31) found that in the seeds of various water 

 plants the protoplasm is not dormant but that the delay is ca\ise)d 

 hy the seed coats. He readily found germination if the coats were 

 broken or removed. 



Oocker asserts that delayed gei'mination, or failure to germinate, 

 is more generally due to seed coat effect (limiting or entirely excluding 

 water or oxygen supply) than to embryo characters as has generally 

 been assumed. Kieszling (32), ShuU (33), Atwood (34) and Hoff- 

 mann (35) have the same conception about this aspect of 

 delayed germination. According to Atwood, it is possible to accept 

 either that the embryo in the course of after-ripening decreases 

 its demands for oxygen, whereby the seeds become able to grow 

 in gases poor in oxygen, or that there is no decrease in oxygen 

 demands but rather an increased permeability of the coat to 

 oxygen. 



(3) Seeds in which an inhibitory partial pressure of carbon dioxide 

 in the tissues of the embryo is the catise of delayed germination. 

 Kidd (36) considers the resting condition of the seed in apparently 

 suitable conditions of temperature, moisture and oxygen supply, 

 as a phase of auto-narcosis under the action of the carbon dioxide 

 produced by the seed itself. Kidd and West (37) interpret the 

 phenomenon of secondary dormancy as a decreased power of the 

 embryo, during the primary period of inhibition in the presence of 

 carbon dioxide, to respond to growth conditions and to germinate 

 under the limitation of the seed-coat. 



(4) Seeds in which the presence of acetic aldehyde inhibits 

 the germination, as is demonstrated by Maz6 (38). 



(5) Seeds in which the expanding embryo meets at the seed coat 

 a mechanical resistance greater than the growing force of the embryo. 

 Crocker and Harrington (28) mention the case that an initial rapid 

 water absorption ceases, before the imbibitional and osmotic forces 

 of the embryo are satisfied, because the swelling of the seed contents 

 is not sufficient to break the seed-coats. 



(6) Seeds in which the coat may exclude chemical compounds 

 necessary for germination, as is supposed by Croker (31c). Brown (39) 

 has found in this connection that the grains of cereals are inclosed 

 within a semi-permeable or selective covering, which permits the 

 passage of water to the interior of the grain, but which prevents the 

 passage of various acids and salts of metals, when they are in aqueous 

 solutions. 



