Dixon and Atkin.s — -Osmotic Pressures in Plants. 449 



liquid air method and the previous one in which the tissues were pressed 

 without preliminary treatment. 



Individual variation in plants belonging to the same species is brought 

 out in the table. Evidently tlie specimen Se has usually a somewhat higher 

 osmotic pressure than Sw, and tlie sap of the latter is in turn considerably 

 more concentrated than that of the tree growing in the Botanic Garden. That 

 these differences may be largely or entirely due to the external conditions is 

 shown by the two observations on September 9th in Sw. Here the exposed 

 leaves had a pressure of 24'4 atm., while those growing in a shaded position 

 had only 19'4atm. The difference between the concentrations observed in 

 Se and Sw may be due to the fact that the leaves were gathered in each 

 case about 10 a.m., and consequently the illumination of the tree growing 

 in the eastern aspect immediately before gathering was moi-e intense than 

 that of the tree exposed to the western sky. Although less variable with 

 conditions of illumination, tlie electrolyte content also is possibly dependent 

 on other external conditions. Thus we find the two plants Se and Sw growing 

 close togetlier in similar soil have similar electrolyte concentration, and this 

 concentration differs from that of the tree grown in the Botanic Garden, 

 where the soil is more moist, and conditions are not so favourable to 

 evaporation. 



Ilex aquifoUum : Leaves. 



The tree which was used in the observations on Ilex aquifolium was 

 a well-grown plant about six metres high growing in an open place in 

 Trinity College Botanic Garden. 



Previous observations (3) on sap pressed from unfrozen leaves showed 

 that we might expect a variation in osmotic pressure in the leaves of the 

 succeeding growths. /. aquifohum usually makes two growths in the year. 

 All the shoots do not elongate at each period of growth, and on some 

 shoots the opening of the buds is much delayed. The leaves remain 

 attached during four or five such periods. Owing to the erratic nature of 

 the elongation, it is impossible to be certain of the exact age of the leaves on 

 the older growths. The observations recorded in Table II are classified 

 into those made on the leaves of the ultimate, penultimate, antepenultimate, 

 and pro-antepenultimate growths. Those on the ultimate growths are 

 further subdivided into mature and immature leaves. Only the experiments 

 made on mature ultimate leaves, or on the previous growths of rapidly 

 elongating shoots, are plotted in the grapli (fig. 2). The numbers of these 

 experiments are marked with an asterisk in Table 11. 



