THE MOVEMENTS OF FLUIDS IN STEMS. 209 



wounds in the stem, we have a slightly different process. It 

 depends on the returning activity of the roots, when the tempera- 

 ture of the soil rises sufficiently to permit the absorption of water, 

 and as no transpiration is going on, from the want of leaves, the 

 " sap " is conveyed to the different parts where growth is going 

 on, to the cambium layer and to the buds. When the substances 

 exuded by trees when bleeding are examined, they are found to 

 be, not pure water, but to be exceedingly rich in most important 

 reserve materials, as starch, sugar, and other substances, such as 

 salts of vegetable acids, all most important to the life of the plant. 

 The material is soon used up, and the cells of the xylem become 

 empty by the time the leaves are rapidly transpiring, and the 

 rapid current in full action. 



Modified stems, as rhizomes, bulbs, and tubers, are best known 

 as reservoirs of starch and other nutrient matters, but the massive 

 stems of trees contain much reserve matter, both in the xylem 

 and phloem. 



V. The growth of the stem. 



It is unnecessary to say anything regarding the growth of 

 young stems, hence we shall confine our remarks to the massive 

 trunks of forest trees. Growth may be best defined as being an 

 increase of volume and permanent change in the form of a plant 

 or part of a plant, due to the intussusception of new nutrient 

 materials, the increase in volume and the change of form occurring 

 together. The circumferential growth of the stem depends on the 

 activity of the cambium layer. Growth of the stem begins early 

 in each season at the expense of the nutrient matters stored up in 

 the stem, and after the exhaustion of this store the new con- 

 structive matters are supplied directly from the nearest leaves, 

 the assimilated matters being thus partly used at once, and partly 

 stored up for future use. The circumferential growth is chai'ac- 

 teristic of the stems of both Dicotyledons and Conifers. During 

 growth the parts are rapidly supplied with water, as growth is as 

 dependent on a proper supply of water as on a proper supply of 

 nutrient matters. Hence, during growth the young cells in the 

 new wood have abundance of protoplasmic contents, and the cell- 

 walls are completely saturated with imbibed water, or by the cell- 

 sap permeating the walls. Then, again, the less indurated parts 

 of the stem, the so-called sap-wood, those conducting the rapid 

 current of water, have the walls fully saturated with water. As 



VOL. VIII., PART III. p 



