564 ' LECTURE XXXIII. 



experimentally, by determining the dry weight of growing parts of various ages'. 

 By this means he obtained the following results, agreeing with what has just 

 been said. 



'In a growing shoot or internode, the percentage amount of the contained 

 water continually increases from the youngest internodes to those which are 

 older, up to a maximum, and then gradually diminishes again;' this decrease occurs 



'only when the growth in 

 length ceases, and the organ at 

 the end of this growth contains 

 most water;' i.e. the decrease 

 of the water per cent, at the 

 conclusion of growth in length 

 coincides with the thickening 

 of the cell-walls which now 

 makes its appearance, and in 

 part also with the deposition of 

 dissolved or solid matters in 

 the cell-contents. It need only 

 be Added that all this is true 

 not merely of the internodes of 

 the shoot, but in exactly the 

 same way of the leaves, and of 

 the transverse zones lying one 

 behind the other in a root- 

 fibre. The quantity of water 

 which young organs may have 

 gained at the end of their 

 growth in length and surface 

 is shown particularly clearly 

 by weighing seedlings, in the 

 fresh and dry states, at the 

 time when all the reserve- 

 materials have been consumed 

 by growth; it is then found 

 that such young plants of the 

 Beet, Bean, Maize, and other 

 species sometimes contain only 

 4 — 5 % dry weight, and 

 similarly, completely developed Mushrooms and submerged water-plants are also 

 found to be extraordinarily rich in water in the later stages of growth. I have 

 found further that pith (parenchymatous tissue) from the interior of various plants, 

 e. g. Senecio umbrosus, which at first contained 4-23 "/o of solid substance, when 



FIG, 3S2.— Parenchyma cells from the median layer of the cortex of the 

 root of Fritiltaria imperialis (longitudinal sections X 5So). A very young 

 cells situated close above the apex of the root, still without cell-sap. £the 

 same cells about smm. above the apex ; the cell-sap s forms isolated drops 

 (vacuoles) in the protoplasm /.plates of protoplasm lying between them. C 

 the same cells about 7 — 8 mm. above the apex ; the two cells to the right 

 below are seen from in front, the large cell to the left below in optical 

 section ; the cell to the right above has been opened by the section, and 

 its nucleus is affected by the penetrating water and exhibits a peculiar 

 swollen appearance. 



' Gregor Kraus, ' t}ber die Wasservertkeilung in der Pflanze^ in Festschr. der naturforsch. Ges. 

 in Halle, 1879. 



