46 



INTRODUCTION TO CYTOLOGY 



system of muscle fibers (Fig. 11, C). In ciliated forms they run beneath 

 the rows of cilia. 



Contractile vacuoles, which exercise an excretory function, originate 

 in the ectoplasm, although later they may lie much deeper. 



A sensory function is performed by the "eyespot," which is sensitive 

 to light, and also by the flagellae and cilia, which are often receptors of 

 tactile stimuli. The eyespot seems in some instances to be plastid-like 

 in character, and will be discussed in Chapter VI. 



Protoplasmic Connections. The fine protoplasmic strands (Plasmo- 

 desmen) connecting many plant cells through pores in the intervening 

 walls are extensions of the ectoplasm (Fig. 12). Several early workers 



A B " C 



FIG. 12. Protoplasmic connections in vascular plants. 



A, B, Pinus pinea: cells of cotyledon. X 375. (After Gardiner and Hill, 1901.) C, 

 Phytelephas ("vegetable ivory"): endosperm cells with greatly thickened walls (w), 

 showing spindle-shaped bundles of connecting strands, m, middle lamella. Semidia- 

 grammatic. 



suspected the presence of such connections before they were able to see 

 them, and even the coarse strands passing through the sieve plates of 

 sieve tubes, though often observed, were not well known until the time 

 of Hanstein's work in 1864. The finer strands of other plant tissues 

 where described in a large number of researches between 1880 and 1900. 

 Among these may be mentioned those of Wille (1883) and Borzi (1886) on 

 the Cyanophyceae; Kohl (1891), Overton (1889), and Meyer (1896) on 

 the Chlorophycese; Hick (1885) on the Fucacese; Hick (1883), Massee 

 (1884), and Rosenvinge (1888) on Floridese; and, on vascular plants, 

 those of Tangl (1879), Russow (1882), Strasburger (1882, 1901), Goros- 

 chankin (1883), Terletzki (1884), Wortman (1887, 1889), Haberlandt 

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

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



With respect to the origin and development of these connecting 

 strands very little is accurately known. Some observers have claimed 

 that the pores through which they pass are present from the time the 

 primary wall is first formed, no wall substance being laid down at these 

 points. Gardiner (1900) believed them to arise directly from the median 



