4 6 



METABOLISM 



Fig. 10 represents a small specimen of Penicillium with its horizontally 

 placed hyphae imbedded in the substratum, and its erect conidiophores pro- 

 jecting into the air. For our present purpose we may imagine the organism 

 simplified so as to be represented by the model illustrated at Fig. n, where 

 the cells of the filament, marked A, B, C, lie in the substratum, whilst the 

 others, a, b, c, &c., are aerial. If now the cell a loses water in consequence of 

 transpiration, it will at once endeavour to obtain a fresh supply from cell b. 

 But b is also transpiring and will, in turn, abstract water from c. That another 

 cell higher up than b is also endeavouring to obtain water is equivalent to 

 imagining that b is transpiring more vigorously ; in other words, to the abstract- 

 ing power of b exerted on c we must add the abstracting power of a also, and 

 so on until we reach the cells which absorb the water from the medium. In 

 these cells the converse process takes place to that exhibited by the transpiring 

 cells. The sucking forces of all the cells, a, b, c, &c., are acting on A, while 

 A replaces the water which it, in consequence, loses by abstracting water not 

 only directly from the medium but also from the cell B. In this way the suck- 



Fig. 10. Small plant oUPenicilliuiH. (After 

 BREFELD.) Contr., conidiophores; Co. % 

 conidia. 



Fig. ii. 



ing action of transpiration is transmitted backwards to all the cells which 

 perform the function of roots. 



As yet we have spoken only of those water currents in the organism which 

 arise from the disturbance of osmotic equilibrium, and which continue to exist 

 so long as there are differences in degree of concentration between the individual 

 cells. If a large number of cells become united to form a simple or a branched 

 cell thread or cell mass, we might conclude that any loss of water in consequence 

 of transpiration from the superficial cells would be made good by water absorbed 

 from the cells lying below. Looked at by itself this assumption would be correct 

 if only the rapidity of the water currents due to osmotic activity were sufficiently 

 great. Even although the pits found in the cell-walls aid in the passage of 

 water, this is true only under special conditions and in relatively small cell- 

 systems such as, for example, in Penicillium. Indeed the mode of occurrence 

 of moulds in nature affords us most valuable suggestions on this subject, for we 

 know that these organisms are confined to situations possessing a certain degree 

 of atmospheric humidity ; we find that they prefer to live in small enclosed 

 areas with stagnant atmospheres, and are quite unable to thrive where there 

 is abundant aeration, even although all the other conditions necessary for their 

 well-being are fulfilled. In these plants the absorption of water by osmosis takes 

 place so slowly, that if the evaporation of water be increased even very slightly 

 the transpiring cells are unable to obtain from below sufficient supplies to 

 compensate them for what they have lost, and the plants in consequence perish 

 from drought. 



Under these circumstances one can very well understand that movement 



