770 



PLANT GROWTH 



500 



-6 -5 



Log cone, of lAA 



Fig. 6. Response of three types of Scor~onera tissue to 



indoleacetic acid. Solid line, normal tissue ; dashed line, 



accoutume tissue; dotted line, tissue from Crown gall. 



(From Gautheret, 1950). 



towards the beginning of prophase, then falls steeply, to remain low during cell 

 division, Meiosis and mitosis both show this cycle. The concentration of ascorbic 

 acid also shows a cyclic change, rising by nearly 50% during mitosis (Stern and 

 Timonen, 1954-5). The synthesis of deoxyribosenucleic acid (DNA), in these an- 

 thers, as evidenced by the incorporation of ^^P into it, goes on at specific periods, 

 mainly just before the meiotic 

 prophase, and in the inter- eiooo- 

 phases before mitosis; the 

 amount of DNA measured 

 microchemically doubles at 

 each of these times. Synthesis 

 of ribosenucleic acid (RNA), 

 however, mainly follows that 

 of DNA, coming in the interval 

 between the end of DNA syn- 

 thesis and the onset of prophase, 

 which fits poorly with the con- 

 cept of RNA as precursor of 

 DNA (Taylor and McMaster, 

 1954). All these changes sug- 

 gest that energy is built up and 

 vised in synthesis, before the 

 nuclear reorganization takes 



place, and that during the process — perhaps because mitochondria or other par- 

 ticles become involved as well, — it cannot so easily be provided. In other words, 

 the dividing nucleus may be metabolically isolated. The evidence that the nucleus 

 contains no cytochrome oxidase or cytochrome C reductase (although it can 

 reduce DPN and TPN) suggests that little energy is available to the nucleus from 

 its own sources (Stern and Timonen, 1945). 



Of the stimuli to cell division, three factors have been mainly studied: auxin, 

 wounding, and special substances. A major part is played by auxin. Cambial 

 division caused by auxin was demonstrated as early as 1935 by Snow in Helianthus 

 and in 1936 by Soding in poplar twigs. The cell multiplication around a wound 

 in trees is partly under the control of auxin, since it varies with the activity of 

 buds higher up, and can be greatly stimulated by auxin application. In tissue 

 cultures, parenchyma of many sorts are stimulated to divide by auxin, and 

 Gautheret pointed out (1942-3) that in general low auxin concentrations cause 

 division, higher ones promote root formation and still higher ones promote cell 

 enlargement and tend to inhibit division. The optimum concentration of lAA for 

 cell division in chicory root tissue cultures is about o. i mg/1. Potato tuber is stimu- 

 lated to divide by 2,4-D but not by lAA; artichoke responds well to lAA. Indeed 

 plant tissue cultures in general were only made possible by the use of auxins. 



A remarkable change in the behavior of tissue cultures with respect to auxin 

 was discovered in 1 942 by Gautheret. After the culture has been maintained on an 

 auxin-containing medium for a number of transfers, it becomes able to grow 

 without auxin (see Gautheret, 1946; Morel and Gautheret, 1947). Carrot root. 



