240 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Sieve-Tubes.* — Continuing his investigation of sieve-tubes, E. de 

 Janczewski Las now made an exhaustive comiiarison of their structure 

 in the different primary sections of the vegetable kingdom, with the 

 following general results : — 



The elements of sieve-tubes are always prismatic in form, more or 

 less elongated, and truncated transversely or obliquely. Their walls 

 are composed of pure cellulose, and are never lignified ; they always 

 have a larger or smaller number of pores, which either permanently 

 retain this structure (in vascular cryj)togams), or soon become per- 

 forated and transformed into true sieves (phanerogams). The mature 

 elements are sometimes empty (gymnosperms), sometimas they contain 

 a parietal layer of granular protoplasm (vascular cryptogams and 

 angiosperms) ; the nucleus is always wanting. 



In vascular cryptogams the elements of the sieve-tubes are no 

 longer than those of the parenchymatous tissue. They contain pro- 

 teinaceous globules adhering to the parietal protoplasm, and collected 

 below the pores. The lateral and terminal walls are furnished wdth 

 a larger or smaller number of pores. The membrane of these pores 

 is never perforated, and presents an obstacle to the intercommunication 

 of the contents of adjoining elements ; it is generally homogeneous, 

 and composed simply of cellulose ; it is sometimes pierced by callose 

 cylinders (as in Pteris aquilina). The sieve-tubes are not influenced 

 by the time of year, but maintain the same condition during their 

 whole existence. 



In gymnosperms the life of the sieve-tubes may be divided into 

 two epochs, evolutive and passive. During the first period the pores 

 situated in the walls of the young tubes produce callose substance, 

 and are transformed into sieves covered and closed by callus ; at 

 this period the elements of the tubes contain parietal protoplasm, 

 and resemble those of vascular cryptogams. During the second 

 period the tubes are totally destitute of protoplasm, and are con- 

 sequently inert ; but there is a communication between adjacent 

 tubes, the sieves losing their callus towards the close of the evolutive 

 period. 



In dicotyledons the structure is more complicated, and the life of 

 the sieve-tubes may be divided into four periods : — evolutive, active, 

 transitory, and passive. During the first period the cambial cell is 

 not transformed directly into an element of the sieve-tube, as in gym- 

 nosperms ; it always divides longitudinally, and produces on one side 

 an element of the sieve-tube, on the other side a mother-cell of the 

 liber-parenchyma, or companion-cells, usually two in number. In 

 the elements thus formed the pores of the walls or the whole of the 

 horizontal septa are covered with a callose substance, and become 

 perforated into true sieves composed of a delicate network of cellulose 

 and a callose envelope. From this moment the sieve-tubes have 

 entered their active period, characterized by the sieve-structure, and 

 by the intercommunication of the protoplasmic contents of adjacent 

 elements. This period sometimes lasts for months or years. In 

 some cases (AristolocJna SipJio, Fagus, Tilia, Bosa) the sieve-tubes in 



* Ann. Sci. Nat., xiv. (l.'!82) (6 pis.). Cf. this Journal, ii. (1882) p. 370. 



