The Initiation and Growth of Tulip Bulbs

Tulip mother bulbs contain daughter bulbs, and second-generationdaughter bulbs are initiated within these daughter bulbs, fromFebruary to July. Initiation takes place progressively, thefirst-initiated bulbs being in the axils of the outermost scales.The innermost second-generation daughter bulbs are finally initiatedat the same time as the flower within the daughter bulb. Thelatest-formed bulbs grow much more rapidly than the others,and soon become larger than the first-formed ones. Later inthe season, growth of daughter bulbs falls into three phases,corresponding roughly with autumn, winter, and spring; growthis rapid in autumn and spring, and slower in winter. The ratesof daughter bulb growth in the autumn are affected by the amountof suppression by the mother bulb apex, but later these differencesdisappear. The durations of the phases of growth also vary withthe position of the daughter bulb. The outer daughter bulbsgrow for a longer time during the autumn phase than the innerones, so that differences in size due to variation in growthrate are partly nullified. In early spring, all daughter bulbsresume rapid growth simultaneously. Final bulb weight is thenapproached asymptotically, with the innermost bulbs having thehighest weights.     

Priority effects among young‐of‐the‐year fish: reduced growth of bluegill sunfish (Lepomis macrochirus) caused by yellow perch (Perca flavescens)?

1. When available, Daphnia spp. are often preferred by age‐0 yellow perch and bluegill sunfish because of energetic profitability. We hypothesised that predation by age‐0 yellow perch could lead to a midsummer decline (MSD) of Daphnia spp. and that priority effects may favour yellow perch because they hatch before bluegill, allowing them to capitalise on Daphnia spp. prior to bluegill emergence. 2. Data were collected from 2004 to 2010 in Pelican Lake, Nebraska, U.S.A. The lake experienced a prolonged MSD in all but 1 year (2005), generally occurring within the first 2 weeks of June except in 2008 and 2010 when it occurred at the end of June. MSD timing is not solely related to seasonal patterns of age‐0 yellow perch consumption. Nevertheless, when Daphnia spp. biomass was low during 2004 and 2006–2010 (<4 mg wet weight L^?1), predation by age‐0 yellow perch seems to have suppressed Daphnia spp. biomass (i.e. <1.0 mg wet weight L^?1). The exception was 2005 when age‐0 yellow perch were absent. 3. Growth of age‐0 bluegill was significantly faster in 2005, when Daphnia spp. were available in greater densities (>4 mg wet weight L^?1) compared with the other years (<0.2 mg wet weight L^?1). 4. We conclude that age‐0 yellow perch are capable of reducing Daphnia biomass prior to the arrival of age‐0 bluegill, ultimately slowing bluegill growth. Thus, priority effects favour age‐0 yellow perch when competing with age‐0 bluegill for Daphnia. However, these effects may be minimised if there is a shorter time between hatching of the two species, higher Daphnia spp. densities or lower age‐0 yellow perch densities.