EXUDATION or WATER FROM UNlNJL'llKD PAUTS. 267 



overhanging bank ffom the sun. The root was then followed 

 I'loin the trunk to the distance of ten feet, wheie it was carefully 

 cut off one foot below the surface, and a piece removed fioiu 

 between the cut and the tree. The end of the root was en- 

 tirely detached from the tree and l3ing in an horizontal position 

 at the depth of one foot in the cold, damp earth, unreached by 

 the sunshine, and for the most part unaffected by the temper- 

 ature of the atmosphere, measured about one inch in diameter. 

 To this was carefully adjusted a mercurial gauge April 26th. 

 The pressure at once became evident, and rose constantly with 

 very slight fluctuations, until at noon on the 30th of April it had 

 attained the unequalled height of 85.80 feet of water." ^ 



717. Pfefi"er'^ attributes the tendency of water to pass only 

 inwards into the woody tissues wholly to the fact that upon that 

 side of the cells whicli faces the interior of the root the osmotic 

 capacity is greater. Within the plant the cell-walls are never 

 saturated with pure water ; but the imbibed liquid is different on 

 different sides, and hence the plasma membrane in contact with 

 the sides must have different capacities for osmosis. 



718. In midwinter or in earliest spring some of the tissues 

 of ligneous plants are stored to a large extent with starch and 

 other solid products manufactured during the previous season. 

 At the coming of warmer weather chemical changes take place, 

 largely following the absorption of water, by which these solid 

 substances are transformed into a liquid state, occupy a gi-eater 

 space than before, and of course exert much greater pressure. 

 The saccharine sap of the maple represents that which dur- 

 ing the early winter existed in the tissues as starchy matter. 

 This conversion of material will be further discussed under 

 '■ Metastasis." 



719. Exudation of water from uninjiired parts of plants. Un- 

 der certain circumstances water can exude in a liquid form from 

 uninjured i)arts ; for instance, through chinks or rifts in the leaf- 

 tips of many monocotyledonous plants, and through water-pores 

 of dicotyledons, especialh' when these are j'oung. Musset ' 

 reports eighty- five drops of liquid falling in one minute from 

 the tip of a leaf of Colocasia esculenta. Duchartre* gives the 

 following figures : Twent^'-five drops fell in one minute from 



1 Ksport of the Secretary of the Mass. Board of Agriculture for 1873, p. 189. 



2 Paanzeuphysiologie, i., 1881, p. 170. 

 8 Comptes Reiidus, Ixi., 1865, p. 683. 



* Ann. des Se. nat. hot., s^r. 4, tome xii., pp. 247, 250. 



