Dixon and Atkins — Osmotic Pressures in Plants. 385 



Concentration of total Sap Solutes. 

 The general form of the curves for the concentration of the wood sap in 

 the stem of the deciduous trees (excepting Ootoneaster frigida, which will be 

 considered later) examined is similar, viz. : — There is a cusp in the early 

 spring, followed by a I'apid fall.^ Tlien a period of low concentration in 

 the summer and early autumn,' followed by a rise, which is gradual at first, 

 and then becomes steeper as it approaches the spring maximum. So far as 

 the investigation goes it shows that tlie concentration of the wood sap in the 

 root follows that of tlie stem, and is generally lower than it, but there does 

 not appear to be such a pronounced rise in tlie spring, and the succeeding fall 

 is not so rapid, so that during the vernal decline in concentration it is often 

 found that the concentration of the wood-sap of the root is greater than that 

 of the stem. Sometimes this difference persists until the concentration of 

 wood-sap of the stem begins to rise again, e.g. Acer, Populus, and possibly 

 Fagus. 



The few observations made upon Salix indicate that the concentrations in 

 the root and the stem are closely similar. 



In contrast to tlie curves traced by the concentrations of the wood-sap of 

 most of the deciduous trees is that exhibited by the sap of the only evergreen 

 examined, viz.. Ilex aquifoUum. 



Here the graph for the concentration of the stem-sap has two cusps — 

 one in summer and one in winter, and corresponding depressions in spring 

 and autumn.^ The relation of the concentration found in the root to that 

 of the stem is reversed, for while in deciduous trees the concentration of the 

 wood-sap is generally greater in the stem than in the root, the converse was 

 found to hold in the case of Ilex. Only in winter was it less in the root 

 than in the stem. During the rest of the year it was greater. 



Cotoneaster frigida is also exceptional in having two cusps in its curve — 

 one in spring and one in autumn. This is doubtless connected with the fact 

 that it is a sub-evergreen. Young examples, up to twenty years old or so, 

 keep their leaves on into the winter. In old specimens, which are deciduous 

 like the tree examined, the opening of the buds in spring is curiously 

 prolonged. 



1 This sudden rise and fall during the spring in the concentration of the carbohydrates of the sap 

 has been detected by Schroder (10) in his investigation of the bleeding of trees. 



2 A. Fischer (3) also found that there was less glucose in the vessels of several trees in the summer 

 and autumn than in the spring. 



3 It is difficult to correlate the variations in concentration in the sap of Ilex with the periods of 

 bud expansion. The period for bud expansion appears uncertain, especially in pruned or lopped 

 trees; thus in the season 1912-1913 buds expanded from November to January. In 1914 the 

 summer buds were opening as early as April 22, and expansion was complete about the beginning of 

 June. On the whole, however, most buds open during the winter and summer months. 



