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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Seeding Practices 



Grass seedling morphogenesis directs attention 

 to seeding practices that are important to germ- 

 ination, emergence, and establishment. Germina- 

 tion tends to increase with planting depth, de- 

 pending on moisture supply and aeration in 

 species which do not require, light for germina- 

 tion. However, to assure good emergence, planting 

 depth should not exceed the normal reach of the 

 coleoptile grown in dark. 



Plant establishment, excluding the effect of 

 disease, is primarily a matter of the extension 

 of adventitious roots. The urgency of adventitious 

 roots depends on the number and longevity of 

 seminal roots, which may remain active for sev- 

 eral months in some species and for less than 

 two months in others (40). The placement of 

 crowns near the soil surface in the process of 

 emergence may preclude the growth of adventi- 

 tious roots at the crown and eliminate any chance 

 of survival in semiarid conditions (25). Type A 

 seedlings commonly place the crowns near the 

 soil surface. In fact, the crowns can be elevated 

 beyond the soil surface to near the surface of a 

 sawdust or other opaque mulch laj'er. Subsequent- 

 ly, deterioration of the mulch can leave the crowns 

 completely exposed. 



Since excessive elevation of the crowns is un- 

 desirable, mulch layers applied immediately after 

 seeding should be very shallow in depth or 

 should admit enough light to the soil surface to 

 stop the crowns below the soil surface. Mulching 

 and cultivating practices are not common in grass 

 seeding. But we surely have missed some impor- 

 tant points of emphasis by failing to take a 

 good look at seedling morphogenesis. Verj^ thin 

 moisture barriers on the soil surface, such as an 

 asphalt emulsion mulch (3), might be very help- 

 ful in obtaining germination and emergence with- 

 out excessive elevation of crowns. The thin mois- 

 ture-barrier type mulch also would contribute to 

 the initial extension of adventitious roots, but 

 other practices should be evaluated. The natural 

 elevation of crowns by some species exposes the 

 seedlings to harsh conditions near the soil sur- 

 face and causes high seedling mortalities. Perhaps 

 most of all, we need cultivating equipment and 

 experimental results on the effects of burying 

 the crowns soon after emergence. Anj' mulching 



applied after emergence might be beneficial, but 

 cultivation is emphasized because soil is the 

 most economical and durable material. "Where es- 

 tablishment of grasses is difficult, such cultiva- 

 tion might be essential to good seeding success. 

 "Where establishment is not difficult, cultivation 

 might nevertheless be beneficial to plant develop- 

 ment and production because the process of tiller- 

 ing requires continuously favorable conditions for 

 the extension of adventitious roots from the 

 crowns of new tillers. 



Shoot Morphogenesis 

 The Shoot Apex 



The grass shoot apex is a prime object in man- 

 agement because it determines the course of subse- 

 quent growth and development. It can be de- 

 stroyed, or it can be protected and influenced by 

 management practices. 



The shoot apex consists of an apical cone with 

 an apical meristem at the summit. Cells at the 

 extreme tip constitute the promeristem. from 

 which the meristems giving rise to fundamental 

 tissues are derived (H). Ridges arising alter- 

 nately below the summit are the first external 

 evidence of differentiation — that is, the forma- 

 tion of leaf primordia. A leaf primordium is at 

 first a slight horizontal ridge that quickly en- 

 circles the apical cone. Three to six leaf primordia 

 are common; but leaf formation lags to such 

 an extent in some species that the number of leaf 

 primordia gradually increases (H)- As leaf 

 growth begins, a leaf primordium expands very 

 rapidly to form a shoulder below the shoot apex. 

 Then the leaf arises vertically from the shoulder 

 to enclose the shoot apex. Leaf growth is at 

 first apical and marginal; however, cell division 

 is restricted to the base of the leaf at an early 

 time. Before the blade attains much length, a 

 collar develops and separates the intercalary 

 meristems of blade and sheath (13). Meristematic 

 specialization proceeds to the interior of the axis 

 where the intercalary meristem of the internode 

 forms above the nodal plate with potential for an 

 axillary bud. 



At the time of transition to reproductive phase, 

 the shoot apex elongates very rapidly and estab- 

 lishes a large number of leaf primordia (5, H). 

 Spikelet primordia, appearing as protrusions in 



