578 LECTURE XXXIII. 



and the denser autumn wood thus formed. It is not improbable that other causes 

 also co-operate here ^ of which, however, we know very little at present ; but in any 

 case the very extensive investigations of De Vries (1872-76) have shown" that 

 my supposition was in the main correct. He intensified the cortical pressure in 

 spring by binding cords tightly round certain spots on two- to three-year old 

 branches ; the result was that the thickness of the annual ring beneath the ligature 

 was less than the average thickness of the corresponding ring at some distance above 

 and below that place. In some branches the difference was so marked, that the 

 place experimented on was visibly thinner — an impression which was strengthened 

 by the formation of woody cushions at the upper and lower margins of the 

 ligature, the latter evidently because at these places cortical tension must have been 

 lessened by the pressure of the ligature. Moreover, the thickness of the layer of 

 autumn-wood, which (as usual) ceased to grow in August, was greater beneath the 

 ligature than in the normal case. The autumn-wood at this place, in very different 

 species (e.g. Maple, Willow, Poplar, Horse-chestnut, &c.) investigated by De Vries, 

 was formed of wood-fibres with the transverse section compressed radially, and the 

 number of vessels was smaller than in the normal wood. These observations show 

 then, that under increased pressure the formation of autumn-wood begins at a time 

 when, during normal growth, wood-tissue with wide cells and many vessels would 

 still be formed. A diminution of cortical pressure is obtained by splitting the cortex 

 into several parts by means of radial longitudinal sections. The strips of cortex thus 

 produced contract somewhat in the tangential direction, because they were previously 

 extended in this direction ; necessarily, the pressure which the cortex exerts on the 

 cambium and young wood is thus diminished, and this most in the immediate 

 neighbourhood of the margins of the cuts ; the new wood-tissue now arising close to 

 these deviates considerably from the ordinary structure of wood. Incisions were 

 made in June and July, after the formation of the normal autumn wood had already 

 commenced ; by the middle of August it was seen that the two- to three-year branches 

 experimented upon had grown considerably more in thickness at the places where 

 longitudinal incisions had been made, than above and below. On transverse sections 

 the thickness of the wood-growths was greatest in the neighbourhood of the inci- 

 sions, and thence diminished constantly up to the middle line between two incisions. 

 De Vries found in all cases that the newly-formed w'ood was outside the layer of 

 autumn-wood which had already been formed before the experiment ; therefore 

 everything produced after the diminution of the pressure was composed of wood- 

 fibres not at all compressed in the radial direction, and at the same time the vessels 

 in this wood were more numerous — i.e. a layer of wood had been formed under 

 diminished pressure, which resembled the spring-wood more than the autumn-wood. 

 By means of these experiments also Knight's old experiments (1801) find their 

 explanation. He fixed young apple-trees, with stems about an inch in diameter, in 



* E. Russow, in his fundamental investigation, ' Über die Entwicklung des Hoftüpfels, der 

 Membranen der Hohzellen und des Jahresj-inges bei den Abietineen, in e7-ster Linie von Pinus 

 silvestris' (134 Sitzung der Dorpater Naturforscher-Ges., Dec. 24, 1881), expresses himself differently 

 from the view given in the text as to the causes of the difference between spring and autumn wood. 



^ Hugo de Vries, 'Über den Einflus's des Rindendruckes auf den anatomisclicn Bau des Holzes," 

 Flora, 1S75, No. VII; and, further, 'Über Wundholz,' Flora, 1876. 



