PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



101 



C. E. Hubbard, Helipterum floribundwn DC, and 

 several species of Cruciferae. Species Avith high 

 frequency in arid tussock-grass grasslands are 

 Iseilema vaginiforum Domin., Portulaca spp., 

 Boerhavia diffusa L., Brachyachne convergent, 

 Eragrostis tenellula (Kunth) Steud. &nd Sid a s~pp. 

 Where the rainfall is higher, some of these spe- 

 cies may be perennial. 



Characteristics Of Annuals 



Annuals are adapted to areas with low annual 

 rainfalls and high frequencies of hot dry periods. 

 Because they depend entirely on seeds for sur- 

 vival and regeneration, factors influencing flow- 

 ering and seed formation are more critical than 

 for perennials. Earliness of flowering is advan- 

 tageous in dry areas, but there appears to be no 

 close correlation between maturity gradings of 

 subterranean clover strains and lengths of grow- 

 ing season (36) ; a number of strains varying in 

 maturity gradings may be found at the one site 

 (27). 



The composition of communities of annual spe- 

 cies may differ from year to year, depending on 

 rainfall-temperature relationships at the start of 

 a new growing season. Composition depends also 

 on chemical and physical properties of the soil, 

 morphological and physiological characteristics 

 and relative numbers of the disseminules present, 

 rates of growth and morphological development 

 of the species that germinate, their reaction to 

 grazing, and production of viable seed. 



Spiny, awned and pointed floral structures ad- 

 hering to caryopses, inflorescences enveloping 

 seeds in a burr and very small seeds or caryop- 

 ses would seem to aid germination on the surface 

 of uncultivated soils (19, 21, 22, 41, 42). Con- 

 versely, seeds and fruits without spiny structures 

 frequently germinate poorly in the absence of cul- 

 tivation and are more likely to be eaten by ani- 

 mals and harvested by ants (21, 41). 



The ecological success of an annual depends 

 in considerable degree also on barriers to ger- 

 mination in periods when chances of seedling sur- 

 vival are low. Annual legumes including subter- 

 ranean clover, barrel medic and Townsville stylo 

 may have a high proportion of their seeds with 

 hard impermeable coats so that a succession of 

 germinations is possible (10, 27, 36). In these cir- 



cumstances death of seedlings from early germi- 

 nations may not be critical for survival of the 

 species. Most annual grasses germinate over a 

 wide range of temperatures and a majority of 

 seeds produced at the end of one season germinate 

 more or less simultaneously at the beginning of 

 the next growing season. 



Some of the best adapted naturalized annuals, 

 barlej - grass and soft brome (Bromus mollis L. 

 (22)), and Bromus rigidus Roth. (36) have dis- 

 tinct but short periods of dormancy and appear to 

 depend for success on single peaks of germina- 

 tion, high densities of seedlings, and capacities to 

 produce seeds quickly despite unfavorable condi- 

 tions for growth. Similarly, freshly harvested 

 seed of Wimmera ryegrass is mostly dormant, but 

 dormancy declines during summer and by late 

 autumn a 90-percent germination is possible (G. 

 A. Pearce, 1971 pers. comm.). 



Growth And Morphological Development 



The developmental morphology of the tiller 

 bud or shoot apex in grasses has been described 

 among others by Sharman ( 38) , and Cooper and 

 Saeed (13). Briefly, the shoot apex .is composed 

 of an apical meristem with the youngest cells 

 near the extreme tip. Leaf primordia arise in 

 the shoot apex and eventually ensheath it; the 

 number of leaf primordia formed varies with 

 species, and Sharman (38) has classified shoot 

 apices as long, intermediate, and short according 

 to the number of their leaf primordia. 



After a certain number of leaves have been 

 produced — the number varying with species and 

 to a degree with environmental conditions (13) — 

 the shoot apex becomes responsive to tempera- 

 ture and to photoperiod. A tiller that has pro- 

 duced its minimum number of leaves will change 

 from vegetative to reproductive growth when its 

 temperature and photoperiodic requirements are 

 met. At this stage, often referred to as "becom- 

 ing ripe to flower" (13). formation of leaf pri- 

 mordia is suppressed and spikelet buds are pro- 

 duced. The differentiation of spikelet buds is ac- 

 companied or followed shortly by elongation of 

 internodes, elevation of the shoot apex and even- 

 tually, emergence of the inflorescence (4)- 



A quantitative description of the growth of an 

 annual grass is contained in Williams' (47, 48, 



