242 SUMMARY OF CURRENT RESEARCHES RELATING TO 



It is sometimes developed throtigli the whole extent of the stem (Buscus, 

 Acorus) ; sometimes it occurs only at the nodes {Convallaria) ; some- 

 times it is found at the base of the stem (Antholyza, Asphodelus, 

 Crocus). This network is in connection, on one hand, with the lower 

 termination of the common bundles; on the other hand, with the 

 vascular and liber-plates of the root. In a second group of mono- 

 cotyledons, those with a variable growth in thickness, the dictyo- 

 genous layer, instead of losing its activity immediately after the 

 development of the roots, as in the first class, preserves its activity for 

 a time. The dictyogenous layer then sometimes (^Aloif) developes 

 several plates of anastomosing bundles, on which the adventitious 

 roots are inserted at varying depths ; its activity then extends as far as 

 the protective sheath or endoderm. Sometimes [Agave) the dictyo- 

 genous layer retains its active condition through the whole life of 

 the plant, and incites the formation of a great mass of fibrovascular 

 bundles, thus favouring the formation of a large number of adven- 

 titious roots. In Draccena and Yucca the dictyogenous layer is re- 

 placed by a secondary meristem which determines the formation of 

 the bundles without relation to the leaves ; on these are inserted the 

 adventitious roots. The structure of the Pandanaceae and Palmas 

 prevents the formation of this radiciferous network ; in these plants 

 the dictyogenous layer organizes the roots ; the fibrovascular bundles 

 penetrate more or less deeply into the central body, and meet the 

 common bundles. 



Two kinds of stem may be distinguished in monocotyledons, those 

 with and those without roots. The latter, supporting the organs 

 of reproduction, and often the leaves, never possess the power of 

 growth in thickness. Their structure is very constant, and is 

 characterized by the existence, at the boundary of the cortex and 

 central body, of an external sheath formed by the lignification of the 

 external layers of the central body, and forming the chief organ for 

 support in the stem. Stems provided with roots are very variable in 

 their structure, and are characterized by the existence of a dictyo- 

 genous layer, and by the presence of the endoderm formed from the 

 internal cortical layer. They may increase in thickness temporarily 

 (Aloe, Ajncia), or permanently (Yucca, Draccena) ; but the stems of most 

 monocotyledons (Pandanaceae, some Palmae, Liliaceae, Irideae, &c.) have 

 no such power. The generating zone or ring of growth described by 

 some anatomists, is ouly the dictyogenous layer, the function of which 

 is to develope the roots and the fibrovascular system which connects 

 the root with the stem. 



Generation of Heat by Arum italicum.* — G. Kraus has tested 

 the elevation of temperature which accompanies the unfolding of the 

 spathe and the opening of the flowers of Arum italicum, and found 

 that where a thermometer was placed in the midst of five opening 

 spathes it rose from 17"- 7 to 44°- 7 C. The heating commences at the 

 apex of the spadix, where it is most considerable, and proceeds down- 



* Abhandl. Naturforsch. Ges. Halle, xvi. (1882) (2 pis.). See Bot. Oentralbl., 

 xii. (1882) p. 224. 



