98 



BOTANY 



PART 1 



penetrate the whole thickness of the cell wall (Fig. 105). The 

 existence of these connecting filaments of living substance between the 

 protoplasts confers an prganic unity on tlie whole body of the plant. 



The members which make up the sieve vessels, or, as they are 

 commonly called, the sieve-tubes (^^), are united by thicker strands of 

 cytoplasm, which facilitate the transfer of substances through the 

 tube. The transverse Avails traversed by these strands of cytoplasm 

 have been referred to above (p. 75) as sieve-pits, the plasmodesms 

 of which are further perforated by mucilage strands. The whole 

 complex of sieve-pits is known as a sieve-plate. In Angiosperms the 



B 





j,r u 



«r 



Im 



— c 



M 



Fi<i. 106. — Parts of sieve-tubes of Citnirbita I'epo, hardened in alcohol. A, Surface view of a sieve- 

 plate ; B, C, longitudinal sections, showing segments of sieve-tubes ; D, contents of two sieve- 

 tube segments, after treatment with sulphuric acid ; s, companion cells ; u, mucilaginous con- 

 tents ; pr, peripheral cytoplasm ; c, callus plate ; c*, small, lateral sieve-pit, with callus plate. 

 (X 510.) 



mucilage strands of each pit ultimately fuse to form a single strand 

 of considerable diameter (Fig. 106). It is worthy of sijecial note 

 that, despite the fact that the nuclei of the sieve-tube segments dis- 

 integrate, the cytoplasm, which lines the wall, remains alive. The 

 walls of sieve-tubes are always unlignified. Their sap cavities con- 

 tain a watery, and more or less dilute, solution of albuminous sub- 

 stances, and small starch grains are also present in most cases. As a 

 rule, the sieve-tubes remain functional only through one vegetative 

 period. Before their activity ceases the sieve-plates become covered 

 (Fig. 106 C) with highly refractive callus (p. 77), Avhich interrupts 

 the conduction of material from one member of the sieve-tube to 

 another. If the sieve-tubes resume their activity in the next vegeta- 



