Hydroids in the Moss Stem and Leaf
Hydroids are almost always thin walled, elongate cells that have diagonaland porous end walls. Click here for an excellent page discussing hydroids and their function and the comparison to vascular plants
They are often used taxonomically to separate genera and species, especially in the Pottiaceae. They are often difficult for the beginner, because it is somewhat difficult to cut the cross sections, and the central strand can degrade and leave an empty space, especially when sections are cut to look for them. Other cells can approach the shape of hydroids in section, further confusing the issue.
It is important to realize that the hydroid strand can die out in a stem, especially near the apex of the stem. This can confuse anyone, especially because there is no convention as to where to cut the stem to look for it. This problem crops up in the BFNA Grimmia key.
When hydroids are found in a moss stem, it is called a "central strand". The blue cells in the longitudinal section to the right constitute a central strand. Central strands can be very difficult to diagnose. Sometimes they appear as a suddenly differentiated bundle as in the drawing of a cross section of a stem on the left, but often the cell size differentiation is somewhat gradual. Note however, that the thickness of the cell walls in the strand is markedly different. Remember that hydroids have very thin walls.These walls often degrade, and the strand just appears as a hollow in the center of the stem:
Both drawings © P. M. Eckel, used by permission 
Here are 2 photos of stem cross sections showing clear central strand. Note in the photo on the left how there is some gradual reduction of the diameter of the cells as you move toward the center of the stem. Still, you can see that the center cells are different. The photo on the right has a more abrupt change, but also note that this is no perfect cylinder of hydroids, and in this stem, all the internal cells are thin walled. Still it is easy to see the different type of cells.
Below is a photo of a stem without a central strand. Note that there is no change at the center of the stem, that is, all cell sizes are roughly the same, and there is no change in the cell wall thickness.
Compare the photo above to the photo below. The one below is what one often sees when one tries to cut stem sections. In other words, it is kind of messy! But look at the cells in the center. They are smaller, and more irregular in shape compared to the cells in the rest of the stem. Some of the cell walls have degraded and disappeared too. So we can say that the stem below has a central strand, but it is weak one. This cross section is indicative of how hard this can be sometimes.
Hydroids in the moss leaf are found in the costa. The structure of the hydroid cells is the same in both stem and leaf, so the keys to recognizing them are similar. In moss leaves, the hydroid strand is usually smaller, so it often has degraded and appears to be a sharply angular hole in the costal cross section. The presence or absence of a hydroid strand in the costa is a central part of species separation in the genus Syntrichia, so most of the photos below will be from that genus. At the end of the page, there are a couple of photos of hydroids in other
genera and families. Here are three photos of leaf cross sections showing a hydroid strand in the costa. In all cases you will notice the very large guide cells, and the very thick-walled stereid cells below them. You will see the hydroids in the notch below the two central guide cells above the stereids. (see the blow up on the right) Notice the very sharply angular gap in this notch. In these photos, the thin hydroid cell walls are still apparent, but that is not always true.
When you look at the photo below, the stereids are not as thick-walled, but the hydroid strand is in the same position. It is much harder to be sure though! The sharp angular opening is the same.
Below are two leaf sections without hydroids. Notice the thick-walled stereid right where the other hydroid strands have been.
