ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 217 



Transitory Conducting Tissue. — The account opens with a general 

 discussion of sieve-tube tissue, which hints at the deep phyletic interest 

 attaching to this line of research. In the Ferns, a relatively primitive 

 group, we find an advanced type of sieve-tube ; and the author is inclined 

 to regard the simpler type of phloem found in certain Lycopods as the 

 result of reduction in descent. The sharp distinction often made between 

 the phloem of Angiosperms and that of other groups is not altogether 

 justified — e.g., the case of companion-cells in Equisetum, etc. The pro- 

 blem before us is the search in the ontogeny of plants for simple organs 

 which function as sieve-tubes temporarily in meristematic regions ; this 

 may afford a guide in the wider phylogenetic problem. 



It is in the Gymnosperms that the author finds such organs, and from 

 this standpoint the group is to be regarded as primitive. In Gymno- 

 sperms the first true sieve-tube elements appear further from the growing- 

 point than the first xylem elements, in contrast with other groups in 

 which the reverse obtains. This results from the presence of the transi- 

 tory tissue (phlohne precurseur), the tubes (tubes precurseurs) comprising 

 which show all stages of differentiation from very simple types (radicle 

 of Thuja orientalis) to complex (Abies Pinsapo). In all cases, however, 

 the tissue is essentially transitory, and, excepting in the root, it exists 

 only in the earliest stages of the ontogeny ; the degradation of the tissue 

 is initiated by loss of turgidity in the cells, which finally become completely 

 absorbed. The details of the nature and arrangements of the phloeme 

 precurseur are given for radicle, hypocotyl, and cotyledons, for Thuja 

 orientalis and Abies Pinsapo. It is unrepresented elsewhere in the life- 

 history, excepting in the meristem of the developing rootlets, in which 

 the arrangement is the same as in the radicle. The primary phloem proper 

 may also be transitory in the seedling, as is well seen in Cryptomeria 

 japonica. Transitory xylem is described in the seedling of Lycopersicum 

 esculent urn ; in this plant the primary sieve-tubes persist. The occur- 

 rence of transitory xylem and phloem in the adult plant is investigated 

 in very young leaves of Abies bracteata, in which first the earliest phloem 

 elements, then the xylem vessels, disappear. Their place is taken by 

 conjunctive parenchyma, so that the vascular bundle, originally single, 

 becomes divided into two by this parenchymatous invasion. The author 

 urges that in all Firs which have a double bundle in the adult leaf the 

 double condition is produced in this same way ; and this is demonstrated 

 in detail also for Pinus Pinea and P. sylvestris, in which the two bundles 

 are widely separated. 



The author then discusses the frequency of transitory conducting 

 tissue in Angiosperms, referring to his previous work on the subject, 

 and citing the cases of Raphanus sativus, Phaseolus vulgaris, Labiate, 

 €henopodiacea3, Triglochin palustris, Liliaceas, Zingiberaceas — in fact, in 

 the most diverse groups of Phanerogams. Finally, he examines in detail 

 the arrangements of the vascular system in the seedling of Mercurialis 

 annua, tracing its course from root-meristem to the tip of the cotyledon. 

 He connects the manifold transitions in disposition of the bundles in 

 th : s course by the existence and disappearance of transitory xylem and 

 phloem, which thus play an important part in the distinction, hitherto 

 regarded as essential, between the vascular structures of root and stem ; 

 and he points out that the same investigation holds good in the case of 

 numberless other plants, whether Angiosperms or Gymnosperms. 



