GLIDING GROWTH AND THE BARS OF SANIO 
group form an unbroken bar at maturity is not conclusive proof that 
the cells of such groups could necessarily not have undergone gliding 
growth; for it seems likely that the differential gliding growth takes 
place chiefly at the ends of the cells. The evidence recently given by 
Klinken, Neeff, and tha. found in apple appear to add sufficient 
direct proof to that formerly published to show beyond a doubt that 
gliding growth occurs among the differentiating wood cells of trees. 
The fact that pits in wood are often found to open into intercellular 
spaces instead of meeting corresponding pits in neighboring cells also 
shows that they are not closed even when cellular displacements have 
deprived them of their former relations or functions. On the other 
hand, the fact that unpaired pits are not of more frequent occurrence 
seems to argue that Jost's assumption to the effect that displaced pits 
induce the formation of corresponding pits on contact with the walls 
of other cells, is probably often the case, except perhaps in instances 
where the unpitted walls encountered are too old. Neeff 's observa- 
tions also support such a view. 
The tissue fusions resulting from budding and grafting also give 
support to the notion that pit connections that have been ruptured 
during gliding growth are subsequently resumed with the new neigh- 
bors on the completion of the process. However, it seems that actual 
contact with other tissues is not always necessary to induce the forma- 
tion of pits, for many cases were found in proliferating tissues of apple 
and pear stems in which the proliferated cells were not in contact 
with other cells and yet contained pits. Vochting^^ has made similar 
observations in Brassica. 
Pits are often penetrated by very fine cytoplasmic threads that 
Strasburger^^ called Plasmodesmen. Such connecting threads have 
recently also been found to exist between the components of so-called 
graft hybrids. Since cytoplasmic connections occur very generally 
between plant cells associated in tissues, it appears that they must 
frequently be ruptured in places where gliding growth occurs. That 
being the case, it seems unavoidable that thin cytoplasmic smears 
1^ Vochting, H. Untersuchungen zur experimentellen Anatomie und Pathologic 
des Pflanzenkorpers. Tubingen, 1908, pp. vii -f- 318, pis. 20. 
Strasburger, E. Ueber Plasmaverbindungen Pflanzlicher Zellen. Jahrb. 
Wiss. Bot. 36: 493-610. 1 90 1. 
Hume, M. On the Presence of Connecting Threads in Graft Hybrids. New 
Phytologist 6: 216-221. 1913. 
