ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 241 



this condition are not affected by changes of season ; in others ( Vitis, 

 Tecoma) the sieves close up before winter, and open again in the 

 spring, from the swelling and contraction of the callose substance 

 which covers them. During the whole of the active period the tubes 

 contain protoplasm, a larger or smaller quantity of a proteinaceous 

 mucilaginous substance (as in Tilia, Vitis, Fagus, &c.), and sometimes 

 starch (Vitis, Tecoma, Fagus). The transitory period usually lasts 

 only for a short time, and is indejDendent of the time of year ; in 

 Vitis and Pyrus it commences in the autumn. During this period the 

 elements of the sieve-tubes gradually lose their contents ; the sieves, 

 previously closed by callus, again open, from the complete absorption 

 of this substance. They have now entered the passive period, aro 

 destitute of all organized contents, and totally inert ; the sieves being 

 reduced to a delicate network of cellulose. 



In monocotyledons the development and behaviour of the sieve- 

 tubes is precisely the same as in dicotyledons ; their existence may be 

 divided into four periods. The fibrovascular bundles are, however, 

 closed, having no cambium-zone capable of producing new sieve-tubes 

 to replace those that have become passive. The active period, there- 

 fore, lasts, as in vascular cryptogams, so long as the life of the organ 

 requires them. The passive period is, in fact, manifested only in very 

 old rhizomes, as in Phragmites. In our climate the active tubes 

 possess, like those of Vitis, the power of closing the sieves in autumn 

 and opening them again in spring. The elements of the active sieve- 

 tubes contain no starch nor any mucilagirous substance ; nor does 

 their parietal protoplasm contain any proteinaceous globules, which 

 appear to be absorbed in spring, and to contribute to the density and 

 refrangibility of the protoplasm. 



Sieve-tubes have, therefore, not the same structure in all vascular 

 plants; it becomes more and more complicated, more and more 

 characteristic, and more and more adapted to fulfil a physiological 

 function, with the elevation of the plant in the scale of organized 

 beings. 



Adventitious Roots of Monocotyledons.* — A careful study made 

 by L. Mangin of the origin and insertion of the adventitious roots of 

 monocotyledons has led to the following resxilts : — 



In all monocotyledons the adventitious roots originate, in accor- 

 dance with the constitution of the tissues of the stem, in a special 

 meristem formed from the peripheral layer of the central body. The 

 central body and the cortex of the young root only appear to be 

 formed from this meristem ; its cap is derived from the internal 

 layers of the '^cortex. This meristem, which the author calls the 

 dictyogenous layer, developes also a special system of fibrovascular 

 bundles intermediate between the root and the stem. This system 

 varies in its arrangements in different groups of monocotyledons. 



In the first and largest group, with annual or perennial, aerial or 

 underground stem, these bundles constitute a network which always 

 occupies the periphery of the central body, the radiciferous network. 



* Ann. Sci. Nat., xiv. (1882) pp. 216-363 (8 pis.). 

 Ser. 2.-^VoL. III. * R 



