360 



LECTURE XXII. 



cited at the beginning ^ ; and the very configuration of the parts of the fruit and 

 seed allows in many cases the relations between plastic matters and growth to be 

 perceived particularly clearly. I must here omit the detailed enumeration of these 

 matters, only finding space for the remark that the materials for growth conveyed 

 to the flowers and young fruits are obviously carried, in green plants which possess 

 foliage at the same time, from the assimilating leaves through the flower-stalks. 

 Nevertheless, the shooting of Tulip and Hyacinth bulbs in spring shows that even 

 in the reservoirs of reserve-materials substances serving for the formation of flowers 

 may be stored up ; and the same is true of ordinary fruit-trees, Horse-chestnuts, 

 etc., the flowers of which unfold in the spring before or simultaneously with the 

 appearance of the green leaves, employing for that purpose the plastic materials 

 reserved in the wood and cortex. 



We now come to the further question, by means of what mechanical arrange- 

 ments are the materials put in motion at the time of growth ? It is simply a matter of 

 the transport of particles the inertia of which has to be overcome, and which must be 

 set in motion by particular forces. Here again, however, the facts are not so simple 

 that they can be rendered clear in a few words. At any rate two essentially diff"erent 

 mechanical processes have to be distinguished from one another, inasmuch as under 

 certain circumstances pressure is able to effect the normal movements of material, 

 though of course as a rule it is the forces of diff'usion which are in play. 



Regarding in the first place those phenomena where pressure comes into con- 

 sideration as the cause of movement, the sieve-tubes and laticiferous vessels may 

 first be mentioned. 



The albuminous substances contained in the sieve-tubes are but little suited for 

 movements of diffusion, hence the transverse and longitudinal walls of the sieve-tubes 

 are provided with fine perforations, and there is no doubt whatever that though these 

 pores are so extremely fine, very considerable quantities of these substances can be forced 

 through them by pressure. If a fresh stem of the Gourd plant, or other very suc- 

 culent shrub, is cut across, the alkaline contents of the phloem-bundles exude in the 

 form of drops which slowly increase in size, it may be till they become as large as a pea, 

 and then (at least in the Cucurbitacese) coagulate. Now it is clear that these reladvely 

 large masses of albuminous substance could not possibly be contained in the segments 

 of the sieve-tubes which have been incidentally cut across, but that the contents of 

 sieve-tubes far distant from the section must here come into view. Taking into 

 account the anatomical constitudon of the sieve-tubes, this exudation of the contents 

 is only possible by means of some relatively strong pressure, which must be exerted 

 on their walls. Such a pressure actually exists, moreover ; it is brought about by the 

 turgescence of the succulent parenchyma, which strives to extend in all directions. 

 In exactly the same manner as the epidermis in a living shoot is passively distended 

 by the succulent parenchyma (as described in Lecture XII), so also must the soft- 

 walled sieve-tubes be compressed by the parenchyma. Of course so long as they 

 are filled with fluid, in the intact plant, this supports the pressure of the parenchyma, 



^ Details as to the behaviour of the matters forming the cell-wall in fruits are found in my 

 treatise, ' Über die Stoffe, '<.vckhe das Material zur Bildung der Zellhäute liefern,' in Jahib. für wiss. 

 Bot. (1863, pp. 270, &c.). 



