PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



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of sub-coleoptile internode and coleoptile for 

 Type A grass seedlings and that of the coleoptile 

 alone for Type B seedlings. 



Foliage leaves are poorly adapted to force a 

 way upward through the soil above the reach of 

 the coleoptile, because they have a long, soft 

 region of meristematic tissue at the base. This 

 leaf intercalary meristem kinks when the leaf 

 blade encounters much resistance {25). By con- 

 trast, the narrow band of meristematic tissue 

 located at the upper end of the sub-coleoptile 

 internode {4) is well adapted to pushing short 

 coleoptiles through the soil. 



Elongation of the sub-coleoptile internode ele- 

 vates the coleoptilar and higher nodes, from 

 which adventitious roots develop. In short, the 

 depth from soil surface to origin of adventitious 

 roots is determined largely by the length of the 

 coleoptile. Short coleoptiles place the origin of 

 adventitious roots very near the soil surface {11) 

 where they are susceptible to the hazards of ex- 

 treme variations in microclimatic conditions {33). 

 For example, blue grama {Bouteloua gracilis 

 (H.B.K.) Lag.) initiates adventitious roots at 

 an average depth of 2 mm. below the soil surface 

 {25). Since Type B grass seedlings do not have 

 elongated sub-coleoptile internodes, the lower- 

 most adventitious roots may arise near planting 

 depth from or near the coleoptilar node. 



Both types of grass seedlings exhibit some 

 typical variations (fig. 2). In fact Hoshikawa 

 {21) defines 6 types of grass seedlings in terms 

 of the presence or absence of (a) sub-coleoptile 

 internode, (b) seminal lateral roots, and (c) ad- 

 ventitious roots from the sub-coleoptile inter- 

 node. Adventitious roots commonly arise from the 

 sub-coleoptile internode in species of Panicum 

 and Andropogon, which have Type A seedlings. 

 These internodal adventitious roots are adaptively 

 advantageous for seedling survival because they 

 arise at various depths in the soil. By contrast, 

 blue grama depends on a single seminal root of 

 short longevity (.i£, 40). and adventitious roots 

 from the nodes of the coleoptile and plumule. 

 Successful extension of adventitious roots from 

 the crown of the primary shoot of blue grama 

 requires 3 or 4 consecutive days of wet, cloudy 

 weather when the seedlings are 3 to 7 weeks old. 

 A typical variation in Type B seedlings is elong- 

 ation in the intra-coleoptile internode (fig. 2), 



Figure 2. — Typical variations within seedling types : A, 

 a Type A seedling has adventitious roots arising from 

 the sub-coleoptile internode (left) ; B, a Type B seed- 

 ling has an elongated intracoleoptile internode. 



which increases in length with increase in plant- 

 ing depth and soil temperature {29). The first 

 tiller may, nevertheless, arise from the coleoptilar 

 node and establish its crown below the crown of 

 the primary shoot {37). Intra-coleoptile inter- 

 node elongation may include more than one inter- 

 node. Consequently, whorls of adventitious roots 

 can be found at two or more depths in the soil. 

 Elongation in either the sub-coleoptile or intra- 

 coleoptile internode elevates the crown of the 

 primary shoot above planting depth. The crown, 

 a section of stem base in which the nodes are 

 not appreciably separated by internode elonga- 

 tion, is the source of both tillers and adventitious 

 roots. Therefore, its depth of placement in the 

 soil is important to development of the plant 

 {38). Vigorous tillering cannot continue unless 

 each successive tiller can establish its own set of 

 adventitious roots. The potential number of ad- 

 ventitious roots per tiller depends on the number 

 of roots per whorl and the number of nodes in 

 the crown. 



