WAIER-GLANDS IN THE LEAF OF SAXIFRAGA CRUSTATA, 409 
bium remains, and enables points of junction to be esta- 
blished. The last-formed fibro-vascular bundles become 
connected in the same manner with the main lateral 
branches. 
An examination of the thickest part of the midrib, as seen 
in a transparent preparation of the leaf, enables one to make 
out very clearly the comparative ages of the constituent 
elements of which the whole bundle is composed. The 
vessels in which the turns of the spiral thickenings are 
approximated most closely can always be traced to the last- 
formed or youngest lobes, whereas those vessels in which 
they are most widely separated belong to the primary fibro- 
vascular bundle, and between these two extremes there is 
every possible gradation. This also proves conclusively that 
the vessels are capable of growth after the formation of the 
spiral thickening. ‘The xylem and phloem are arranged on 
the collateral type, and the bundle is surrounded by a well- 
developed endodermis. 
Development of the water-gland.—Having thus followed 
out the differentation of the tissues of the leaf, we may 
proceed to follow out the development of the water-glands. 
And since what is true of one gland is true of all, it will 
suffice to study the development of the first-formed or ter- 
minal gland. This can be most satisfactorily done by a 
series of longitudinal sections of the leaf in various stages 
of growth (fig 2). 
As before siated the rudimentary leaf consists of undiffer- 
entiated meristematic cells covered by adermatogen. When 
the differentiation of the procambium bundles has taken 
place, the meristematic ground-tissue cells at its apex divide, 
forming a mass of closely packed polygonal cells with deli- 
cate cell walls and very conspicuous nuclei. This is the 
first indication of what will hereafter be the gland. The 
further conversion of the primary meristem into gland-tissue 
takes place from the point of its first occurrence, towards 
the upper side of the leaf, finally ending beneath the der- 
matogen. At the inner extremity of the gland towards the 
fibro-vascular bundle, and at the lower third of its under 
surface (i.e. the surface corresponding to the lower side of 
the leaf) certain cells are conspicuous as becoming more 
elongated and fusiform than the rest and as subsequently pre- 
senting reticulate thickenings on their walls. They are 
joined end to end, and eventually become continuous with 
the spiral vessels of the fibro-vascular bundle. ‘These cells 
present a series of intermediate forms between reticulate cells 
and spiral vessels. At the same time changes have been going 
