[The regulation of DNA synthesis in fern gametophytes by light]

In young gametophytes (= sporelings) of the common male fern (Dryopteris filix-mas) short wavelength light (= blue light) specifically increases DNA replication and mitotic activity even if the sporelings have the same growth rate under blue and red light, as determined by dry matter increase. Cell number and DNA content of sporelings of the same age and the same dry matter content are much higher under blue than under red light (Figs. 1, 2). In the second part of the paper it is pointed out that the "cell" (or unit of DNA) may not be used as a system of reference for biochemical data such as protein or RNA content in the case of the fern sporelings (and possibly not in many other organisms either). The appropriate system of reference seems to be the entire multicellular system if precaution is taken that the systems grow with the same growth rate under the different experimental conditions (e.g. under blue and red light in the case of the fern sporelings).