ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 477 



Sieve-tube System of Cucurbitacese.* — Dr. A. Fischer classifies the 

 sieve-tubes found in the Cucurbitaceas under four beads : — (1) The 

 vascular- bundle sieve-tubes, found in the sieve-portions of the vas- 

 cular bundles ; (2) hypodermal or ectocyclic, between the epidermis 

 and the stiffening-ring ; (3) entocyclic, within the stiffening-ring ; 

 (4) commissural, those which serve to combine the other kinds of 

 sieve-tube with one another. 



The distribution and mode of formation of the sieve-tubes in the 

 main stem of Cucurbita are described at length. Independently of 

 some ectocyclic tubes which are inclosed in the collenchyma, and 

 have become functionless, a number of small sieve-bundles occur in 

 the cortical parenchyma near to the collenchyma, forming a much- 

 branched and extensive system. The stilfening-ring separates this 

 system in the internode completely {rom. that of the vascular bundles. 

 Within the thickening-ring are ten vascular bundles, each with two 

 sieve-portions, and in addition some distinct entocyclic bundles con- 

 nected with one another by commissural tubes, which also unite the 

 different tubes of each kind with one another. The distribution of 

 the sieve-tubes the author regards as indirectly connected with their 

 function as conductors of albuminous substances, and as determined 

 mainly by the local consumption of albuminoids in the internode. 



The author then treats of the facts connected with the sieve-tubes 

 in other parts of the plant of Cucurbita ; and in particular the gradual 

 change which they undergo in passing from the stem into the root. The 

 ovary possesses an extraordinarily richly developed sieve-tube system, 

 especially in its innermost layer. After impregnation the peripheral 

 layer of the ovary grows most rapidly, and new hypodermal sieve- 

 tubes are formed in it. They are also found in the style close to 

 the central conducting tissue, where they serve, in the opinion of the 

 author, to supply the pollen-tube, during its growth, with albuminoids. 

 He states also that the pollen-tubes branch abundantly, so that one 

 tube can impregnate a number of ovules. 



The author examined 28 species of Curcurbitaceee, belonging to 

 as many difierent genera ; and distinguishes the following six types 

 in reference to the mode of distribution of the sieve-tube system : — 

 (1) Alsomitra ; (2) Luff a ; (3) Bryonia ; (4) Cydanthera ; (5) Lage- 

 naria ; (6) Cucurbita. The first type has only collateral bundles 

 without commissures or peripheral sieve-tubes. In the second type 

 there are bicollateral vascular bundles, but only a few rudimentary 

 peripheral sieve-tubes ; no radial commissures. The third type has 

 numerous entocyclic, but no ectocyclic or commissural sieve-tubes. 

 In the foui'th type the numerous entocyclic tubes have only a few 

 commissures ; the ectocyclic are wanting. The fifth type is distin- 

 guished from the fourth only by a much more abundantly developed 

 commissural network, and leads to the sixth type, where there are also 

 ectocyclic tubes ; and the whole system attains its highest development. 



The author states further that it is only the cells of the young 



* Fischer, A., ' Unters. lib. d. Siebrohrensy.stem der Cucurbitaceen,' 109 pp. 

 (6 pis.) 8 vo, Berlin, 1884. 



