MECHANICAL AIDS IN DIFFERENTIATION OF TISSUES 45 



sufficient for this purpose. The expansion of the tissues themselves, as 

 well as the stresses and strains due to neighbouring actively growing 

 zones, act in the same way in enlarging the intercellular spaces. The 

 large intercellular spaces in the petioles of NympJiaea and Calla are 

 produced in an actively growing tissue, which expands against the resistance 

 of the stretched peripheral tissues, whereas the hollow internodes of 

 grass-haulms are produced by the passive stretching and rupture of pith. 



All uniformly extensible limiting membranes when stretched by 

 internal pressure tend to become spherical, and this applies to the cell 

 as well as to an india-rubber balloon or soap-bubble. Owing to their 

 mutual pressure, however, the cells may become polygonal in homo- 

 geneous tissues, and the varied shapes of unicellular organisms show 

 that in the absence of such mutual pressures, soft cell-walls may diverge 

 very greatly from the spherical outline. In addition, cells of special 

 shape and intercellular spaces of remarkable outline may be produced 

 in tissues 1 . 



The appearance of intercellular spaces renders it possible for the 

 contiguous cells to grow into these spaces, just as the epidermal cells 

 may grow outwards to form hairs. In both cases, however, the process 

 is under the plant's control, and this applies to the formation of the internal 

 hairs in the intercellular spaces of NympJiaea, as well as to the production 

 of the endocarpal hairs in the loculi of the fruits of Citrus. Similarly the 

 formation of tyloses within the cavities of the vessels is a localized 

 phenomenon, and only takes place at a certain age 2 . Most pollen-tubes 

 and the hyphae of many parasitic fungi grow along the intercellular 

 spaces, applying themselves closely to the cell-walls and enlarging the 

 narrower spaces by means of the pressure exercised by the growing apex 

 as it penetrates between the boundary walls. In other cases, however, 

 the tubes or hyphae excrete enzymes which exercise a solvent action 

 upon cellulose, and either loosen the union between the cells or enable 

 the filaments to bore through the cell-walls. 



It is therefore hardly surprising that certain cells should become 

 longer than others of similar origin, and should penetrate between the 

 latter by a process of sliding growth. This commonly occurs to a slight 

 extent and is often extremely pronounced. A good example is afforded by 

 the laticiferous tubes of Euphorbia, which exhibit continued apical growth 

 and send ramifications into each new portion of the tissue in which they 

 occur. Sliding growth also forms an essential condition for the formation 

 of bast and wood-fibres, and indeed of all secondary prosenchymatous 



1 Zimmermann, Beitrage z. Morphol. u. Physiol., 1893, p. 198, and the literature there 

 quoted. 



3 Schellenberg, Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. 261 ; Mellink, Bot. Ztg., 1886, p. 749. 



