390 



CONDUCTING SYSTEM 



purascens). As a rule the entire wall separating two adjacent segments 

 of such a sieve- tube is occupied except for a narrow marginal zone- 

 by a single pit of unusual size, the thin closing membrane (sieve-plate) 

 of which is traversed by fine perforations. These sieve-tubes contain 

 large quantities of " protoplasm " (? slimy protein material). The very 

 characteristic sieve-tubes of the Phaeophyceae were first observed by 

 H. Will in the older parts of the stipe of Macrocystis hwurians 

 (Fig. 157 a). The structures in question occur in radial rows at the 

 periphery of the central core of hyphal tissue. Their horizontal or 

 slightly oblique transverse walls are converted bodily into sieve-plates, 

 with wide perforations, which in the older sieve-tubes become occluded 

 by callus-plates. Their longitudinal walls are moderately thick. The 



A. 



Fig. 157. 



A. Sieve-tube from an old stipe of Macrocystis luzurians. B. Sieve-tubes in the stipe 



of Fucus scrratus. After Wille. 



segments are rather wide and are furnished with a peripheral layer of 

 protoplasm, which is often covered with a slimy, vacuolated substance. 

 Almost contemporaneous with Will's observations were N. Wille's 

 researches upon the sieve-tubes of various species of Laminaria and 

 Fucus. In the former genus, the sieve-tube segments are comparatively 

 narrow, but expand at both ends just like the corresponding structures 

 of Angiosperms. The transverse septa are finely perforated over their 

 entire surface. These sieve-tubes, which contain large quantities of 

 " protoplasm " (? slimy protein material) extend mainly in the longitu- 

 dinal direction, but are connected at frequent intervals by anastomoses 

 (Fig. 157 b). The sieve- tube system of the stipe is continued into the 

 flattened leaf-like portion of the thallus, where it is located in the tissue 

 that intervenes between the two photosynthetic layers. According to 

 Wille and Hanosteen, the sieve-tubes of the Fucaceae closely resemble 

 those of the Laminariaceae as regards both structure and arrangement. 



