90 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Course and Termination of the Sieve-tubes in the Leaves.* — 

 A. Koch has carried out a series of observations ou this subject, of 

 which the following arc the general conclusions : — 



1. At the periphery of the leaf are the blind ends both of the 

 tracheitis and of the sieve-tubes of strong fibrovascular bundles. The 

 tracheids of the delicate bundles which run towards the margin of 

 the leaf are attached at a right angle to those of the sympodial mar- 

 ginal bundle, and do not end blindly ; and the same is the case with 

 the sieve-tubes. In the interior of the leaf there are usually no sieve- 

 tubes in the last ramificatious of the bundles with free ends. 



2. The sieve-tubes of the reticulate bundles are attached directly 

 on all sides to one another, and form no blind cuds. Anastomoses 

 of sieve-tubes between the upper and lower sieve-portions, especially 

 of stronger bundles, open a communication between the two groups of 

 sieve-tubes. 



3. The sieve-tubes never leave the immediate neighbourhood of 

 the tracheids, as the laticiferous tubes do. 



4. All tlie sieve-tubes of the leaves have conducting cells ; the 

 membranes between the two are perforated by numerous pores. 



5. The sieve-tubes of the upper sieve-portion of all delicate 

 bundles are surrounded, in the leaves of all species of CucurbitacesB 

 examined, by green parenchymatous cells, and are in close ajiposition 

 to the wall of the palisade- cells. According to Sachs's hypothesis of 

 the formation of albuminoids in the sieve- tubes, this is a speciality of 

 the upper sieve-tubes which is wanting in the lower sieve-portions. 

 We have here a peculiarity of the Cucurbitaceaj ; in other jdants with 

 bicolliiteral caulino bundles the delicate bundles which run through 

 the green parenchyma have only lower sieve-portions. 



t3. A similar special function is also performed by the perijihcral 

 cells of the lower sieve-portions, which, in the Cucurbitaceaj examined, 

 are very rich in albuminoids, as soon as the loaf is placed in favour- 

 able conditions for assimilation. These albuminoids are very often 

 formed only in those cells whose wall is in direct contact with that 

 of the palisade-cells. 



7. Cells of this kind always accomjiany the rows of tracheids of 

 the inner ends of the bundles up to the last, at least in all the Cucur- 

 bitaccfe examined, and in some other plants. 



8. The sieve-tubes of the delicate bundles of Cncurbitaceaj were in 

 summer found to be almost filled only with thin fluid contents, and 

 to be free from mucilage ; in November, on the contrary, in Echallium, 

 they were almost all full of mucilage. The cause of this phenomenon 

 was not discovered. The same peculiarity was exhibited by the sieve- 

 tubes of the cotyledons of Cunirhita, from winter sowings, at the time 

 when the reserve materials were being transported. 



9. The sieve-tubes of the cotyledons of Cucurbita dried in the 

 ordinary way were closed by thick layers of callus after the dis- 

 appearance of the mucilage. The same phenomena were observed in 

 the sieve-tubes of the stem of plants of Cucurbita kept for a time in 



* liot. Zig., xlii. (1884) pp. 401-11. 417-27 (1 pi.). 



