LECTURE XXII. 



PASSAGE OF THE PLASTIC MATERIALS THROUGH THE TISSUES. 



That the materials serving for the construction of the organs must travel in the 

 growing and assimilating plant over distances more or less extensive, results as a 

 necessary consequence from all the facts hitherto stated. Growth takes place at the 

 apices of the roots on the one hand, and in and beneath the buds on the other, and 

 plastic substances are of course employed there : at the time of germination they are 

 contained in the reservoirs of reserve-materials, and must travel from thence to the 

 places mentioned. In fully-grown plants with assimilating foliage leaves, the latter 

 behave like reservoirs of reserve-materials; the plastic substances necessary for the- 

 growth of the buds and of the distant roots pass out from them. That the 

 path which individual molecules of starch and sugar and the substances forming 

 protoplasm pass over may, under certain circumstances, be very long, is obvious 

 on reflecting that the materials of which the subterranean roots of a tree (e. g. a 

 Palm) 20 metres high are formed, have been originally produced in the crown of 

 leaves, 20 metres or more distant from the roots. 



But it is also easy to convince ourselves of this fact experimentally. If, for 

 instance, the bud at the end of a well-developed Gourd plant is directed through 

 a narrow hole into the dark cavity of a wooden box, as in Fig. 238, it is obvious 

 that all those materials which are now employed in the growth of the parts in 

 the dark must necessarily have been derived from the leaves which assimilate 

 under the influence of light outside the box. The fig^ure represents a definite, 

 observed case. The Gourd plant made use of in the experiment already possessed 

 thirteen large leaves on its main stem, the total assimilating area of which amounted 

 to about i'5 square metres when, on the 25th of July, 1881, the terminal bud of the 

 stem was directed at d into the box K. All the buds and leaf-shoots in the axils of 

 the leaves had been previously cut away, in order that all the products of assimilation 

 of the thirteen leaves should pass into the bud which was now in the dark. Inside the 

 box, which was about two metres high, one metre broad, and one metre long, there 

 developed, in the course of the next four or five weeks, an extremely vigorous system 

 of etiolated shoots, with white stems and leaf-stalks, and yellow leaves. On the ist 

 of September, that is, after five weeks, the terminal bud of the etiolated main stem 

 was conducted to the exterior through a hole e in the roof of the box ; and the shoot 

 which had grown in the dark inside the box, and which possessed structural peculiarities 

 [3] A a 



