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154 plant species, chosen at random, and collected in the Netherlands were investigated cytologically. The chromosome numbers determined were compared with data known from other countries.

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A cytologica l analysis of 11 92 artificia ll y produced hybrids in the co llec ti ve apo-amphimictic species complex, Hieracium pilosel/af H . peleterianum, showed that 16 addition hybrids were formed ; 15 of these were produced by the fu sion of one reduced and one unreduced gamete and one by the fu sion of two unreduced ga metes. Zygotic chro moso me doubling co uld be un ambi guou sly ruled out. Polyspermaty and soma tic doubling in meristema tic ti ss ue of the creeping stolons did not occur. The percentage ( 1·26% ) of unred uced ga metes is low but sufficient for the production of considerable numbers of addition hybrid s, eve n in moderately large population s. The fact th at addition hybrid s are ra re under natural conditions suggests that the adaptive va lue of these hybrids is inferior.

Fluorescence lifetime imaging is a relatively new technique for acquiring directly the nanosecond temporal characteristics of the fluorescence emission of a spatially extended object, and for capturing the dynamic features at every pixel of an image simultaneously. In general, the applications of fluorescence lifetime imaging have been mainly in the microscope, but other diverse imaging situations can benefit from the technology. Our instrument employs periodically modulated excitation light, synchronous modulation of the amplification stages of a microchannel plate intensifier, and subsequent digital recording of the image with a charge-coupled device (CCD) camera. The digitized images can be subsequently analyzed with a variety of different ways. A short description of the lifetime resolved fluorescence imaging instrumentation is given together with typical applications depicting lifetime spatial distributions, multiple lifetime analysis, statistical analysis of the image data, and suppression or enhancement of particular fluorescent species. 1.2 Advantages of temporal resolution of the fluorescence emission. Conventional fluorescence microscopy measures a steady-state fluorescence signal. However, the O-8194-1432-8/94/$6.OO SPIE Vol. 2137 / 105 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx SPIEVo!. 2137/ 107