272 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



leaves develop very early in such plants, so that active leaves are formed imme- 

 diately beneath the terminal growing-point. These leaves withdraw Water, 

 by transpiration, from the terminal bud and thus cause its destruction. If such 

 plants are cultivated in an atmosphere nearly saturated with water vapor, the 

 terminal bud is protected from destruction and the stem develops with mono- 

 podial branching. Various plants that usually have short internodes, such, for 

 instance, as Bellis perennis, Capsella bursa-pastoris, and Sempervivum when cul- 

 tivated in a saturated atmosphere, develop a stem with leaves arranged spirally 

 (Fig. 123). In these cases the reduction of the primary stem occurring under 

 usual conditions is due to a deficiency of water; the rosette of leaves forms 

 rapidly and transpires very actively, thus depriving the terminal bud of ade- 

 quate water supply. 



All these observations and experiments show that the same species may be 

 very definitely modified, in external form as well as in internal structure, by 

 variations in the moisture condition of soil and air, and that the changes thus 

 produced are very striking. The question now arises: Why does the amount 

 of water absorbed by the plant have so great an influence upon its formal 

 development? 



Turgidity, as is well known, is a condition essential to growth. The more 

 water is contained in the plant, the more its cells can be stretched from within. 

 Enlargement is terminated when water ceases to enter the cell. Wortmann 

 found, in experiments with Lepidium sativum, that root-hairs are very long and 

 thin when grown in water, while they remain short and their cell walls are 

 much thickened when they are grown in sugar solutions. The cellulose that 

 produced the increased expanse of cell wall in the first case, produced thickening 

 in the second case. The same thing occurs when the water supply is not suffi- 

 cient for the usual growth of the plant, small cells with thick cell walls being 

 formed in this case also. 



Substances dissolved in water influence the entrance of the solvent into the 

 cells, not only by their osmotic activity, but also by changes that they may pro- 

 duce in the protoplasmic membrane, as was shown by the investigations of 

 Ritter. 1 This author found that both organic and inorganic acids produce 

 striking structural changes in the hyphae of some of the lower fungi, especially 

 the Mucorinae. Giant cells are formed which may be thirty or forty times as 

 large as are the ordinary hyphal cells. The giant cells of Mucor spinosus are 

 typical of these; they are formed when the spores are allowed to germinate in 

 a nutrient medium containing citric, tartaric, or malic acid. 



This phenomenon is due, at any rate, to a change in the osmotic properties 

 of the plasma membrane under the influence of acids. Such a conclusion is 

 supported by the later work of Czapek, 2 who has furnished direct evidence favor- 

 ing the hypothesis that acids may greatly increase the permeability of the plasma 

 membrane, thus facilitating the outward diffusion of substances dissolved in 

 the cell sap. 



1 Ritter, G., Ueber Kugelhefe und Riesenzellen bei einigen Mucoraceen. Ber. Deutsch. Bot. Ges. 25 : 

 2SS-266. 1907. 



2 Czapek, F., Versuche iiber Exosmose aus Pflanzenzellen. Ber. Deutsch. Bot. Ges. 28: 159-169. 1910. 



