46 GROWTH AND CELL-DIVISION 



elements which are distinctly longer than the cambium-cells which produce 

 them l . 



If we suppose that first intercellular spaces are formed, and that then 

 the elongating cells grow into these, the two processes involved are more 

 easily understood, although as a matter of fact they seem always to occur 

 simultaneously. It remains, however, uncertain whether the surrounding 

 cells are forcibly pushed apart, or whether they aid the penetrating cells 

 by separating to make way for them. The shape and growth of the 

 laticiferous tubes is undoubtedly partly determined by the tissues in which 

 they grow, either by the mechanical restriction or chemotropic attraction 

 of the growing tubes to certain paths, or by a combination of these factors, 

 as in the case of pollen-tubes. In the latter case rapid sliding growth is 

 possible without causing any permanent discontinuity in the tissue sufficient 

 to appreciably affect its mechanical rigidity. 



Cells and cell-filaments may unite to form a loose mycelium or even 

 a solid mass of pseudoparenchyma 2 such as is produced by the closely 

 wefted hyphae of certain fungi. In lichens the symbiotic algae and fungi 

 are closely apposed, and fusion may even occur between them. A 

 satisfactory explanation of the factors which produce and regulate these 

 secondary associations would be of considerable general importance, for 

 in sliding growth and in grafting we have instances of the combination 

 of cells and tissues which were originally separate. 



SECTION 14. The Order of Cell-division. 



In the growing regions of vegetative organs, two similar halves are 

 usually produced at each division, and since the walls intersect at right 

 angles, the resulting general arrangement is easily traced. Similarly the 

 nature of the curves which will be produced by the growth of aggregations 

 of similar cells can be predicted beforehand, and in spite of the common 

 occurrence of secondary alterations and displacements, these curves can 

 be recognized in many tissues. Conversely the primary and secondary 

 arrangement in the tissue afford indications as to the mode of division 

 of the cells composing it. Moreover, a knowledge of the conventional 



1 Hofmeister (Pflanzenzelle, 1867, p. 162) first drew attention to the existence of sliding 

 growth, and Krabbe (Das gleitende Wachsthum, 1886, p. 41) discussed its importance in the growth 

 of vessels, &c. Nathansohn (Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 682) has shown that it also 

 occurs between the tracheides and parenchyma of intercalary zones of growth, and between the 

 vascular bundles and cortex of roots. See also Kohl, Mechanik der Reizkriimmungen, 1894, p. 33; 

 and on Rivularia, Schwendener, Sitzungsb. d. Berl. Akad., 1894, p. 958. There are naturally 

 different varieties of sliding growth. 



a Lindau (Festschrift fur Schwendener, 1899, p. 28) suggests the term ' plectenchyma.' Cf. 

 also Bitter, Jahrb. f. wiss. Bot, 1899, Bd. XXXIV, p. 230; Askenasy, Ber. d. Bot. Ges., 1888, 

 p. 130 (Algae). 



