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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Since adventitious roots arise on the shoots, 

 morphogenetic characteristics that cause excessive 

 elevation of shoot primordia also may cause ex- 

 cessive elevation of adventitious-root primofdia. 

 Some of the important phenomena will be in- 

 cluded in the topics on seedling morphogenesis 

 and shoot morphogenesis. 



Seedling Morphogenesis 



Morphological Types Of Seedlings 



For management purposes, we should recog- 

 nize two types of grass seedlings (fig. 1) : Type A 

 typically has an elongated sub-coleoptile inter- 

 node and a short coleoptile ; whereas, Type B has 

 a long coleoptile and no elongation in the sub- 

 coleoptile internode. These seedling types elevate 

 root and shoot primordia in different ways (32, 

 35, 36). 



A 



BLUE GRAMA SEEDLING 



The terms mesocotyl and epicotyl have been 

 repudiated from the terminology of parts of 

 grass seedlings (4). Consequently, I call the first 

 internode of the grass seedling (located below 

 the divergence of the coleoptile) a sub-coleoptile 

 internode, and the second internode (located 

 above the divergence of, and inside, the coleop- 

 tile) an intra-coleoptile internode. 



The coleoptile is the structure of grasses adapt- 

 ed specifically for seedling emergence. It protects 

 the plumule bud within it while forcing its way 

 upward through the soil. The seed of species 

 having very short coleoptiles would need to be 

 planted so shallow that they would be susceptible 

 to the hazards of severe environmental fluctua- 

 tions near the soil surface, unless the seedlings 

 exhibited the characteristic of elongated sub-cole- 

 optile internode. Thus, the reach of the coleop- 

 tile above planting depth is the total length 



B 



CRESTED WHEATGRASS SEEDLING 



ADVENTITIOUS ROOT-___ 



— SUB-COLEOPTILE INTERNODE 



LEAF 



COLEOPTILE 



SEED 



SEMINAL LATERAL ROOT-== 



SEMINAL PRIMARY ROOT 



Figure 1. — Two types of grass seedlings : Type A, the blue grama seedling, has an elongated sub-eoleoptile internode 

 and a short coleoptile ; and Type B, the crested wheatgrass seedling, lacks the elongated sub-coleoptile internode 

 but has a long coleoptile. 



