McEvoy Peter B
Department of Entomology, Oregon State University, 97331, Corvallis, OR, USA.
Oecologia. 1984 Feb;61(2):160-168. doi: 10.1007/BF00396754.
Marginal and central florets of the capitula of tansy ragwort Senecio jacobaea yield different kinds of fruit. The central ("disk") achenes are lighter (x±SE=199±5μg), more numerous (x±SE=58±0.6 achenes per head), and are equipped with a pappus aiding wind transport and rows of trichomes aiding animal transport. The marginal ("ray") achenes are heavier (x±SE=286±7μg), less numerous (virtually invariant at 13 achenes per head), and lack dispersal structures. Whereas disk achenes are relased shortly after they mature, ray achenes are retained by the parent for a period of months following maturity.Germination at constant temperature (20°C) and with alternating light (12 h light: 12 h dark) demonstrated that disk and ray achenes exhibit different germination syndromes. Germination percentage increases linearly with achene fresh weight in both types; for a given weight, disk achenes have a higher germination percentage than ray achenes. Germination time decreases with increasing achene weight in disk achenes, but increases with achene weight in ray achenes.The germination percentages and germination times for disk and ray achenes diverge progressively with increasing achene weight. The divergence in behavior is a result of diverging patterns of dry matter allocation in the two achene types. Increase in the size of disk achenes favors the embryo fraction, thereby speeding germination while reducing protection. Increase in the size of the ray achenes favors the pericarp fraction, thereby increasing protection while delaying germination.Reduced germination percentage and germination speed of the ray achenes were shown by experimental manipulation to be caused by physical inhibition by their thicker pericarps.Dimorphism in ragwort likely speards germination out in space and time, thereby increasing the number of safe sites an individual parent can exploit in disseminating offspring. The syndrome in other heteromorphic composites resembles that of ragwort, generally combining reduced dispersal-delayed germination in the outer achenes and distance dispersal-quick germination in the central achenes. The outer achenes are generally less numerous and larger. Dispersal traits (large numbers, early release and light wieght) are the direct opposite of dormancy traits (small numbers, delayed release and heavier weight). Thus conflicts between the properties determining dormancy and dispersal appear to require separate dormancy and dispersal phenotypes.
千里光属植物欧洲千里光头状花序的边缘小花和中央小花产生不同类型的果实。中央（“盘状”）瘦果较轻（x±SE = 199±5μg），数量较多（x±SE = 每个头状花序58±0.6个瘦果），并具有有助于风力传播的冠毛和有助于动物传播的几排毛状体。边缘（“舌状”）瘦果较重（x±SE = 286±7μg），数量较少（每个头状花序实际上恒定为13个瘦果），并且缺乏传播结构。盘状瘦果在成熟后不久就会释放，而舌状瘦果在成熟后会被母体保留数月。在恒温（20°C）和交替光照（12小时光照：12小时黑暗）条件下的发芽试验表明，盘状瘦果和舌状瘦果表现出不同的发芽综合征。两种类型的发芽率均随瘦果鲜重呈线性增加；对于给定的重量，盘状瘦果的发芽率高于舌状瘦果。盘状瘦果的发芽时间随瘦果重量增加而减少，而舌状瘦果的发芽时间随瘦果重量增加而增加。盘状瘦果和舌状瘦果的发芽率和发芽时间随着瘦果重量的增加而逐渐分化。行为上的差异是两种瘦果类型干物质分配模式不同的结果。盘状瘦果大小的增加有利于胚部分，从而加快发芽速度，同时减少保护。舌状瘦果大小的增加有利于果皮部分，从而增加保护，同时延迟发芽。通过实验操作表明，舌状瘦果发芽率和发芽速度的降低是由其较厚的果皮的物理抑制作用引起的。千里光的二态性可能在空间和时间上分散发芽，从而增加单个母体在传播后代时可以利用的安全地点数量。其他异型菊科植物的综合征与千里光相似，通常在外围瘦果中结合了减少传播 - 延迟发芽和在中央瘦果中远距离传播 - 快速发芽。外围瘦果通常数量较少且较大。传播特征（数量多、释放早和重量轻）与休眠特征（数量少、释放延迟和重量重）正好相反。因此，决定休眠和传播的特性之间的冲突似乎需要单独的休眠和传播表型。
