16 INTRODUCTION TO CYTOLOGY 



adequate means of keeping the activities of the various parts fully 

 coordinated. Among the means by which this correlation of the parts 

 is maintained, we may consider protoplasmic continuity, the differentia- 

 tion of specialized tracts, hormones, and physiological gradients. 



The delicate intercellular strands in the tissues of multicellular plants 

 have been described in a large number of papers. ^^ Two general types 

 of connection are usually distinguished. In the red algse and certain 

 other thallophytes adjacent cells are connected at one relatively large 

 region where membrane materials are deposited. The question of actual 

 protoplasmic continuity through this region is at present a debated one 

 (Jungers, 1933). Large protoplasmic strands are found between the egg 

 and surrounding cells in cycads and in certain other tissues, but these 

 seem to be due either to the enlargement of smaller pores present at an 

 early stage or to actual solution of the intervening wall. Strands of the 

 second general type, which are known as plasmodesms (Fig. 11), are of 

 exceedingly small diameter, special methods being required for their 

 demonstration. They may be distributed rather uniformly over the wall, 

 or they may be aggregated in small groups, which are often located in 

 pits or thin spots; frequently they are branched. 



Very little is accurately known regarding the origin and development 

 of plasmodesms. It can scarcely be doubted that the pores through 

 which they pass are often present from the time the cell partitions are 

 first formed, no wall substance being deposited at these points. There 

 is also considerable evidence in support of the view that plasmodesms 

 are secondarily developed structures. Strasburger (1901) stated that 

 extensions from adjacent cells come into contact as the intervening wall 

 begins to thicken but do not form continuous strands. The fact that 

 in separating cells the break occurs through the thickened median portion 

 of the strands lends support to this view (Hume, 1913). That the 

 strands are actually continuous was emphasized by Meyer (1896 et seq.), 

 who held that they are due both to retention and to new formation. He 

 observed a secondary formation of connections in Volvox and in fungus 

 hypha? which came in contact. 



Light on this question has been sought in parasites and graft hybrids, 

 where cells of different species come together. Kienitz-Gerloff, Kuhla, 



1^ Among these may be mentioned the works of Wille (1883) and Borzi (1886) 

 on the Cyanophycese; Kohl (1891), E. Overton (1889), and Meyer (1896) on the 

 Chlorophyceae; Hick (1885) and Doubt (1928) on the Fucacese; Hick (1883), Massee 

 (1884), Rosenvinge (1888), and Mangenot (1926) on Florideje; Kohl (1897) on mosses; 

 and, on vascuhir plants, those of Tangl (1879), Russow (1882), Strasburger (1882, 

 1901a), Goroschankin (1883), Terletski (1884), Wortmann (1887, 1889), Haberlandt 

 (1890), Kienitz-Gerloff (1891), Jonsson (1892), Kuhla (1900), Poirault (1893), 

 Gardiner (1884, 1897, 1900), Hill (1900, 1901), Gardiner and Hill (1901), Kohl 

 (1900, 1902), and Quisumbing (1925). For general accounts of protoplasmic connec- 

 tions, see Davis (1905a), Meyer (1920), Lundeg&rdh (1922), and Jungers (1930). 



