ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 869 



cells incline -upwards ; and tlie formation of fresh material must 

 consequently take place in tlie opposite to the iisTial direction. The 

 same occurs also in Asdepias curassavica. 



In addition to the contact of the laticiferons tuhes with the spongy- 

 parenchyma, the conduction of food-material is also assisted by special 

 contrivances. In EupJiorhia palustris this is effected by the extension 

 of the spongy parenchyma in a direction at right angles to that of the 

 laticiferous tubes ; in thick-leaved species by the formation of paren- 

 chymatous sheaths resulting from the spongy parenchymatous cells 

 being in close uninterrupted contact with the laticiferous tubes. 



The laticiferous tubes ramify abundantly underneath or in the 

 palisade-layer. Where the tubes accompany the vascular bundle of 

 the leaf (as in E. Myrsinites and Hypochceris raclicata) they put out 

 branches in an upward direction, which often ramify or dichotomize, 

 and the ends of which abut on the palisade-cells. The lateral branch 

 appears to break through the sheath of the vascular bundle, and not 

 unfrequently gives the impression as if some of the cells of the sheath 

 were employed in the formation of the branch. But the history of 

 development shows that they are entii'ely independent. The walls of 

 the laticiferous tubes have no special structure ; here and there pits 

 were found, the thin closing membrane of which appeared to be per- 

 forated by very fine pores. 



The development of the reticulation of laticiferous tubes in the 

 leaves is in inverse proportion to that of the conducting parenchyma. 

 The ends of the vascular bundles project directly into the air-contain- 

 ing intercellular spaces. When the laticiferous tubes are abundant 

 the parenchymatous sheath is imperfect, and its cells alter their form ; 

 they become as broad as long, and of irregular outline, corresponding 

 to their functional degeneration. The so-called parenchyma of the 

 veins disappears with the increase of the laticiferous tubes ; a trans- 

 verse section of the principal vein of E. Myrsinites shows that it is 

 completely wanting, or nearly so. 



In HypocJioeris radicata the lamina of the leaf always has a very 

 strong mid- rib ; but the greatly developed parenchyma appears here 

 to fulfil a mechanical function. 



Medullary Vascular Bundles of some Dicotyledons.* — An ex- 

 tended investigation by J. E. Weiss of the fibrovascular bundles 

 found in the pith of a large number of dicotyledons, has led to the 

 following general conclusions. 



The later origin of the medullary bundles in comparison to that 

 of the larger bundles of the peripheral ring, does not justify the con- 

 clusion that they are cauline (i. e. originate exclusively in the stem). 

 In the Cucurbitacese, Papaver orientale, Actcea foetida, Cimicifuga, and 

 Tludictrum, it cannot, however, be positively asserted that they are 

 not cauline. The bundles in the stem of Statice and Armeria are 

 certainly of common origin. 



In the so-called " endogenous " formation of vascular bundles, the 



* Bot. Centralbl., xv. (1883) pp. 280-95, 318-27, 358-67, 390-7, 401-14 

 (1 pi.). 



