PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



93 



the axils of leaf primordia, often appear first 

 at about the middle of the elongated shoot apex. 

 Further differentiation proceeds upward and 

 downward (31f). The appearance of a spikelet 

 primordium at the lowermost leaf ridge termi- 

 nates the potential for initiation of new leaves 

 on that shoot. 



Environmental conditions before and during 

 the time of floral induction determine the num- 

 ber or proportion of shoot apices that become 

 reproductive, within genetic limitations. Environ- 

 mental conditions during the time of floral dif- 

 ferentiation and growth affect the size of the 

 inflorescence — the number of spikelets. And en- 

 vironmental conditions during the time of an- 

 thesis and seed formation affect the number and 

 size of caryopses. Since the shoot apex and its 

 production of vegetative and reproductive struc- 

 tures is subject to the effects of management 

 practices, the genetic limitations and environ- 

 mental requirements of plants should be known. 

 Perhaps, most of all, one should know the time 

 of floral induction and floral differentiation (12. 

 15,17,19,26,28,30). 



Floral induction is interpreted as a biochemical 

 process that may require a cold pretreatment 

 (vernalization), a certain day length (long day 

 or short day), and a certain sequence of day 

 lengths (long before short or vice versa). On the 

 other hand, some species flower independently 

 of day length. Effects of day length are modified 

 by temperature, and the entire process requires 

 a minimal amount of vegetative development. 

 Mature leaves are needed to feed the process, 

 because assimilates are not exported from im- 

 mature leaves (4-1). Thus, poor vegetative de- 

 velopment or excessive defoliation at the season 

 of floral induction will reduce reproductive de- 

 velopment in most perennial grasses. Fertilization 

 and other practices should be scheduled to coin- 

 cide with a particular phase of development to 

 attain described results. 



The Phytomer 



A shoot is built up by a succession of segments 

 or phyton-units called phytomers (13, 16, 39). 

 A phytomer includes the sequence of structures 

 produced by the meristematic tissues contiguous 

 to a leaf primordium. From top to bottom, a 

 phytomer consists of blade, sheath, internode, 



axillary bud (or potential thereof), and node; 

 however, the node often is omitted from the 

 description (13, 39). The potential for adventi- 

 tious roots generally takes place near the nodal 

 plate. 



The elongation of blade, sheath, and internode 

 appears predominantly as a 1, 2, 3 sequence ; how- 

 ever, the internode may remain very short. One 

 leaf follows closely, and is enclosed by the one 

 immediately older. Thus, the process of vegetative 

 growth is regular and indeterminate. An individ- 

 ual phytomer exhibits a determinate type of vege- 

 tative growth. If the blade is cut when immature, 

 the undamaged intercalary meristem at the base 

 continues to differentiate blade tissue. The leaf 

 tip cannot rejuvenate. If a mature blade is re- 

 moved, subsequent growth will be sheath and 

 culm internode. However, the sheath and inter- 

 node may remain unusually short when the blade 

 of that phytomer is removed (1). 



The growth of grass shoots can be defined and 

 graphed, phytomer by phytomer, to illustrate 

 rhythm. of growth within and among phytomers 

 as affected by environmental conditions or treat- 

 ments (22). 



The Crown 



The term crown is used to mean any section 

 of stem base in which two or more nodes remain 

 close together. It includes the vertical section 

 of a terminal rhizome, which has no special name ; 

 the proaxis of timothy (11) ; and, in fact, the 

 basal part of essentially all grass shoots. The 

 number of nodes in the crown indicates the num- 

 ber of basal leaves produced and the potential 

 number of tillers and adventitious roots. Both 

 the size and number of nodes in the crown deter- 

 mine the degree of elevation of tiller crowns 

 therefrom. Where crowns are large, successive 

 tiller crowns can be initiated at a much elevated 

 position. Some species literally grow themselves 

 out of the soil, eventually leaving dead centers. 

 The most obvious example that I have seen is 

 that of weeping lovegrass (Eragrostic curvula 

 (Schrad.) Nees), but many species exhibit the 

 same phenomena in a more subtle way. Tiller 

 crowns may develop entirely above the soil sur- 

 face. A deep accumulation of litter or dead 

 stubble can contribute to crown elevation and 

 can preclude the development of adventitious 



