310 FRIMARy ARRANGEMENT OF TISSUES. 



history of development remains still to be made. The arrangement can still be re- 

 cognised after considerable secondary thickening of the stem ; the stem-embracing 

 insertions of the branches which are thick, but attached by thin bases, weave in a 

 basket-like manner round the node of the leading axis : it is especially developed 

 in the perennial subterranean shoots of Myrrhis, species of Chaerophyllum, &c. 



INIany Umbellifcrce have, as is obvious from the continuity of the pith-cylinder 

 on both sides, another form of axillary insertion, which remains to be more exactly 

 investigated : thus Silaus pratensis with its medullary bundles noticed on p. 253. 



The same form of axillary insertion as in the above-named Umbelliferae is found 

 in Aralia japonica; it remains to be investigated whether the same is the case in 

 other Araliacese, and in other families in which the leaf-insertion, and perhaps also 

 the bundle-system, resembles that of the Umbelliferse, e. g. Ranunculaceae with alter- 

 nating leaves. 



The above-nientioned fourth, and very special case of bud-insertion occurs in 

 species of Echinocactus, and some of Cereus with thick shoots (C. candicans }). Its 

 development requires investigation. In the mature state the leaf-traces having one 

 bundle are found united to form sympodial bundles, which are separate and per- 

 pendicular, and of equal number to the angles of the stem (Echinocactus), or are 

 connected in a reticulate manner; between them are broad medullary rays. The 

 leaf-bundles run sl'ghily obliquel)', almost horizontally upwards towards the lower 

 margin of the spine-cushion, that is towards the point of insertion of the rudimentary 

 leaves. Just above each foliar-bundle and in a direction almost parallel to it, the 

 thick cortex of the stem is traversed by some few vascular bundles, which are near 

 one another, and have their xylem-portions turned towards one another; .these 

 belong to the axillary bud formed above the rudimentary leaf, and attain a consider- 

 able strength as soon as the bud developes into a shoot. These bundles of the bud 

 now pass through the medullary rays, between the sympodia of the leaf-trace in the 

 stem, into the pilh, and there branch freely in all directions, their branches being 

 united one with another to form an elaborate plexus traversing the whole pith. 

 This system of bundles of the bud is only direcdy connected with the sympodia of 

 the leaf-trace by single short connecting bundles at the points of passage through 

 the medullary rays. In the Opuntias, Cereus speciosissimus, &c., and also in 

 the Rhipsalidacea? ^ this phenomenon is wanting; the bundles of the bud, as far as 

 investigated, are inserted, in the manner usual for Dicotyledons, partly on the cortical 

 bundles, partly on thos^e of the bundle-ring: medullary bundles are altogether wanting. 

 In the Mamillarias, which have medullary bundles (p. 254), no connection between 

 these and the young lateral shoots has as yet beei"^ discovered. 



The above-named plants have accordingly a system of medullary bundles, 

 which differs fundamentally in its significance fiom the others described above on 



P- 253- 



Where these latter, and where cortical bundles occur in Dicotyledons, the 



axillary insertion occurs, as far as is known, in one (usually the first) of the typical 



forms, with the addition of direct connection between the medullary or cortical 



bundles of the leading shoot and of the lateral shoot. 



* Compare Vochting, /.c. (p. 261). 



