158 Saunders . — The Leaf -skin Theory of the Stem: 
Convolvulaceae that the description which has been given for Calystegia 
may be taken as applying equally to the present case. 
Gymnosperms. 
Coniferae. The obviously decurrent character of the leaves in the 
adult shoot of many species has already been mentioned. In a seedling 
plant of Juniper, for example, it is evident at a glance that between succes- 
sive whorls of three leaves the surface of the stem proper is nowhere exposed. 
The three longitudinal contact lines seen in each internode extend through- 
out its length, terminating in the mid-axils of the whorl below. In an 
Araucaria seedling a ‘ pick-up ’ pattern after the manner of a Dicotyledon 
is especially conspicuous, while in Pinus maritima (Fig. 8) and Picea 
orientalis , Carr, as mentioned above (p. 138), we have a hypocotyl with 
ridges and furrows corresponding in number with the cotyledons. 
5. Evidence from the Pteridophyta. 
A discussion of the question whether the leaf-skin theory is generally 
applicable to the Pteridophyta lies outside the scope of the present paper. 
It will not however be out of place to call attention to certain features in one 
or two types which suggest that the same relation may exist among 
Vascular Cryptogams as has been found to run through the Spermaphyta. 
Equisetum . The sequence of cell divisions accompanying the develop- 
ment of the leaf in Equisetum is shown in the accompanying figure, 
which is reproduced from Sachs’s ‘ Lehrbuch ’ (Fig. 273, first edition ; 
Fig. 279 a, second edition). A careful scrutiny of this illustration brings to 
light the fact that the development of the successive segments cut off from 
the same side of the apical cell follows a different course in alternate seg- 
ments. In one rapid divisions take place in an anticlinal and a periclinal 
direction which soon result in the production of a slight bulge (seethe point 
marked b on the right-hand side of the figure). In the segment next above, 
after the first anticlinal divisions have occurred, development for the time 
being comes to a stand. It is further evident from the drawing that a leaf 
is developed from each of the rapidly dividing segments, and that the whole 
product of one such segment (i. e. the cell group between any two segments 
which after one or two divisions remain for a time quiescent) takes part in 
its formation. As the cells forming the bulge or leaf primordium undergo 
further division, one of their number situated at the apex takes on the 
functions of an apical cell, and it is by the segmentation of this cell that the 
elongation of the leaf in an upward direction is continued. The cells form- 
ing the lower part of the bulge, i. e. the residual cells derived from the same 
original segment, also continue to divide and furnish the surface layer of the 
lengthening internode (see cells marked r). At a much later period one of 
the group of cells (marked i) derived from the alternate primary segment 
