394 



CONDUCTING SYSTEM 



the vessels (and tracheides) still contain living protoplasm at a very 

 advanced stae;e of differentiation. 



The development of bordered pits has been studied in detail by 

 Sanio, Kussow and Strasburger, especially in Pinus sylvestris and certain 

 other Conifers. The formation of the border is preceded 

 by the appearance of a wide " primordial pit," in the 

 centre of which the torus is laid down as a circular 

 thickened patch. At the same time the closing mem- 

 brane of this pit bulges towards one side in a peculiar 

 manner, its cross-section at this stage being compared by 

 Kussow to the shape of the Greek letter ^. Shortly 

 afterwards the border is deposited on both sides of the 

 primordial pit. As seen in radial section, the border- 

 membrane first appears as a narrow, sharply defined ring, 

 which rapidly grows in width. This annular membrane 

 extends in an oblique direction towards the interior of 

 the cell, thus gradually contracting the opening which 

 constitutes the pit cavity, until the horizontal projection 

 of its margin coincides with the edge of the torus. The 

 development of the pit is then practically completed. 



The ontogeny of sieve-plates 184 has been carefully 

 examined by Kussow, A. Fischer, Strasburger, Lecomte 

 and A. W. Hill, among others. Each sieve-plate is derived 

 from the closing membrane of a wide, shallow primordial 

 pit, which occupies practically the whole of the transverse 

 wall, if the latter is horizontal or only slightly oblique. 

 The closing membrane of this pit becomes thickened in 

 a reticulate manner. The unthickened portions or 

 " sieve-fields " are pierced by protoplasmic connecting- 

 threads, as was first shown by Kussow. Each sieve-field 

 may be traversed by a single connecting thread (as in 

 Vitis vinifera) or by several (as in Wistaria sinensis and 

 Cucurbita Pepo). Both sides of the sieve-plate then 

 become invested by callus, which stains readily with aniline blue. It 

 is only in Angiosperms that the closing membranes of the sieve-fields 

 disappear ; according to Hill, the obliteration begins around the proto- 

 plasmic connecting threads, which are subsequently converted into 

 " slime-strings." The narrow canals in which the slime-strings are 

 contained gradually become dilated ; if several are present to start with 

 in the same sieve-field, they ultimately coalesce with one another. As 

 a final result, the sieve-fields disappear altogether. 



Fig. 158. 



Young vessel from 

 a primary root of 

 Zea Mais. Pla.smo- 

 lysis shows that the 

 protoplasts of ad- 

 joining segments 

 have not fused. 



