572 LECTURE XXXIII. 



change their volume by pressure and traction by means of the forces which are here 

 acting. Hence we must assume that the strips of tissue which shorten increase in 

 diameter, and that the elongating pith becomes correspondingly thinner. These 

 alterations in transverse dimensions are, however, not directly capable of measurement. 



At any rate it follows from what has been said that the passive extension 

 in length makes the cells of the epidermis, the vascular bundles, and the not yet 

 lignified sclerenchyma strands narrower, and that the epidermis especially, since it 

 is properly too short for the growing parenchyma, must also be too narrow for it. 

 In like manner the pith, prevented by the epidermis from extending, must tend to 

 be distended transversely : since the pith (and soft parenchyma generally) is too long 

 for the passively extended tissues, it must also at the same time be too thick for 

 them, and tend to extend them (particularly the epidermis) tangentially. In other 

 words, it follows at once from the easily measurable tissue-tension in the longitudinal 

 direction of a growing shoot-axis, that transverse tensions also must exist ; or, since 

 the passively extended tissues are too short for the soft parenchyma, they are also 

 too narrow for it, and this conclusion may be directly proved. If thin transverse 

 discs are cut from the organs in question, and split open by means of a radial longi- 

 tudinal section, they gape, because the epidermis contracts peripherally, since it was 

 previously properly too short for the internal tissue — i. e. it was passively stretched. 

 However, we will not here go further into detail as to the changes in form of split 

 transverse discs of growing organs, because it would be impossible to avoid difficult 

 mechanical considerations of various kinds. I will refer only to the one obvious 

 fact, that the increase in circumference of many internodes and leaf-stalks which 

 are subsequently hollow is effected by this extension of the external layers 

 of tissue, while the internal pith is no longer able to grow transversely in pro- 

 portion; the latter therefore becomes ruptured, its outer layers remaining in 

 connection with the external tissues which are growing in circumference, while a 

 cavity arises in the interior. This may be easily observed in the flower-stems of the 

 Teazel {Dipsacus), the scapes of the common Onion and Dandelion {Taraxacum 

 officinale), in many Umbelliferge and true Grasses, &c. 



The cylinder of pith of a dicotyledonous shoot when freed from its enveloping 

 layers of tissue is very limp, extensible and pliant, but if laid in water it soon 

 becomes turgid, stiff, and elastic : it is then longer and apparently also thicker. The 

 elongation in water may in a few hours amount to 40 per cent, or even more of the 

 original length. This proves that the cells of the pith absorb with great force the 

 water surrounding it, and that their cell-walls are still in a high degree extensible, and 

 thus that they did not possess that degree of turgescence, while yet in the interior of 

 the uninjured organ, of which they are capable. Further observations on such 

 isolated cylinders of pith, particularly of Compositae and Solanese (where they often 

 attain a very considerable thickness and are peculiarly suited for experiments of this 

 kind), may be employed for further instructive investigations, of which I will only 

 describe one more in detail here. The isolated pith of a piece of the shoot of Scftecio 

 umbrosus 235-5 mm. long increased in length 5-7 per cent, at the moment of isolation, 

 and weighed 5-3 gr. It was divided into three parts by marks of Indian ink : of these 

 I was the oldest and III the youngest. The lengths were 1= 100 mm., 11= 100 mm., 

 111 = 49-9 mm. 



