SPOROPHYTE. 21 



separate this slit forms a leaf-gap in the adjacent (inner) face of each 

 branch. From the lower margin of both gaps the trough-like leaf -trace 

 comes off, forming for a time a connection across the fork from one branch 

 to the other. It soon becomes distinct from both. Leaf -gaps are easily 

 dissected out, since the tissues readily separate along the line of the endo- 

 dermis. 



As stated above, the cortex and medulla of the stem come into contact 

 or continuity through the leaf-gap (fig. 100). The starchy layers always 

 connect, and sometimes a strand of sclerotic cells connects the outer cortex 

 with the similar central medulla. In about half the nodes examined (9 out 

 of 20) the sclerotic medulla passes the leaf- gap as a solid rod unchanged. 

 In one -fifth (4) it connects directly with the outer cortex through the 

 wide leaf -gaps. In 7 a rod of sclerenchyma passes from the medulla out- 

 ward in the groove of the leaf -trace to vanish in the petiole or to become 

 continuous there with a peripheral layer of similar cells. This rod may 

 originate independently in the starchy medulla shortly below the node, or 

 it may begin as a ridge on the sclerotic medulla, which is gradually con- 

 stricted off. These different arrangements of sclerenchyma may occur at 

 successive nodes of one stem. 



Between the cortex and the xylem of the stem, all of the inner and outer 

 tissues become continuous around the margins of the leaf -gap endodermis 

 with endodermis, pericycle with pericycle, phloem with phloem, and con- 

 junctive parenchyma with conjunctive parenchyma (fig. 108). Or, the 

 phloem may become very thin, or may be completely interrupted for a 

 short distance on one or other side of the gap. A transverse section of 

 the stem through a leaf -gap shows the vascular system as a deep, round 

 horseshoe (fig. 100). No new tissue elements are seen at the nodes. 

 Spiral vessels are not found in leaf -trace or stem. The cells bend out 

 from stem to leaf by gentle curves, without any noticeable peculiarities. 



Where the stem forks each tissue system remains continuous and 

 unbroken (fig. 99). There is no ramular gap. One can best imagine the 

 structure by starting with a Y-shaped object made of round rods, welded 

 together below. Let this represent the sclerotic medulla. We dip the 

 object into melted wax, coating it all over; this represents the layer of 

 starchy medulla. Successive coatings of suitable thickness may represent 

 inner endodermis, inner pericycle, inner phloem, xylem, outer phloem, outer 

 pericycle, outer endodermis, starchy cortex, sclerotic cortex, and epidermis. 

 In the angle of the Y the continuity of tissues from one arm to the other 

 is strikingly smooth and regular. A single scalariform tracheid may 

 extend for a long distance in each arm. On the sides of the fork this con- 

 tinuity involves angular elements of peculiar shapes (fig. 98). 



The apex of the stem is so clothed with hairs as to appear smoothly 

 rounded (fig. 3). Under this covering there is a shallow, basin-like de- 

 pression (fig. 70), not often symmetrical, at the tip of the stem. A low 



