Phycomyces sporangiophores grow away from stationary objects, a phenomenon known as the avoidance response. Evidence is presented suggesting that a growth-stimulating gas is emitted from the sporangiophore and is then swept to the leeward side by air currents resulting in higher gas concentration on that side. The presence of a stationary barrier decreases the passive movement of the gas away from the leeward side. It is proposed that an increase of this gas on one side causes that side to grow faster. Indirect evidence suggests that the gas is water vapor.
During the development of the sporangiophore of the fungus Phycomyces blakesleeanus there occurs a period of several hours when the sporangiophore does not elongate; instead, its "growth" is diverted into the formation of a sporangium at its top. This period of head formation is called stage 1I. Clearly, growth has not ceased but rather the geometry of the growing area has changed from that of a cylinder to a sphere. The AA(t,t + 10) represents the area of the sporangium at time t subtracted from the area of the sporangium 10 min later. The subtraction of the constant, 1, is an arbitrary adjustment in order to make R(t) = 0 when the change in growth is equal to zero. With this function AA for any given 10-min interval can be evaluated with respect to a predicted \A determined by averaging the AA values before and after a 10-min interval. All areas were calculated by treating the head as a perfect sphere, i.e., area = 47rr2. Detailed investigation of the growth location in the sporangium was accomplished by the use of photomicrographs. Photomicrographs were taken with a 35-mm camera attached to a Leitz Ortholux microscope. Stage I sporangiophores were observed under 50X magnification until the onset of stage II. Cornstarch was then gently blown onto the specimen. The photographs were enlarged to 125X for Figure 1 and to 400X when used as a basis for Table I. The location at d (Table I)
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