Group Distinctions
Under one of the more widely accepted classification schemes, there are six different genera of hornworts: Anthoceros, Dendroceros, Folioceros, Megaceros, Notothylas and Phaeoceros. Members of these genera share a lot of the same characteristics, with a few notable exceptions.
The most significant differences can be seen in Notothylas. This genus tends to be yellowish instead of dark green. Their sporophytes, the stalks with their spores at the top, are different as well. In most hornworts sporophytes are tall and thin, resembling a long horn. In fact, that's where they get the name ''hornwort'' (''wort'' simply means ''small plant''). However, sporophytes in Notothylas are short, thick, and slightly curved, and the sporophytes do not grow continuously as they do in other species.
Other differences come from habitat. Members of Anthoceros typically grow on the banks of rivers or other bodies of water, while members of Dendroceros grow on the bark of trees or sometimes in swamps. Dendroceros also differs in shape, growing in ribbon-like branches with wavy edges. Other species of hornworts often grow in a more curled and round shape.
Overall Characteristics
In general, hornworts share many characteristics, including the typically tall and thin sporophyte shape. One similarity is that most hornworts only have a single large chloroplast (which is the green organelle used to capture energy from sunlight) in each cell. By contrast, liverworts and mosses usually have several chloroplasts per cell. The ability of hornworts to store carbon dioxide (CO2) for later use in photosynthesis also sets them apart from liverworts.
Hornworts generally live in wet, humid places. Within this habitat requirement, they can be found on rocks, in soil, or on tree bark, but they do need the humidity to survive. This is because of their lack of vascular tissue as well as the fact that they have no true roots. Instead, they have small, hair-like rhizoids that keep them firmly attached and help with water absorption.
Some Interesting Facts
Hornworts are fascinating plants. For one, they all, regardless of genus, have a symbiotic relationship with a bacteria called Nostoc. The bacteria collects and grows in small pockets inside the hornwort and supplies the plant with nitrogen. In return, the hornwort provides the bacteria with carbohydrates.
Hornwort sporophytes are also an interesting feature. In most species, they can grow to be over 10 centimeters (4 inches) high! The sporophytes are growing continuously in all genera except Notothylas. The growth occurs at the base of the sporophyte stalk and pushes it upwards.
Lesson Summary
All right, let's take a moment or two to review. As we learned, hornworts are a type of bryophyte, along with mosses and liverworts. We also learned that they're non-vascular, meaning that they lack the xylem and phloem tissue that other types of plants have for moving water and nutrients, and there are only about 100 to 150 different species contained within six genera. Notothylas is notably different in color as well as the fact that its sporophytes are short and thick and don't grow continuously. Other differences between genera include habitat, although all hornworts prefer humid areas, typically near water.
Hornworts are different from other bryophytes because they usually have only one large chloroplast per cell, which is the green organelle used to capture energy from sunlight, and they're able to store carbon dioxide. Hornworts have rhizoids instead of true roots, which are hair-like structures that keep them firmly attached and help with water absorption. Interestingly, they have a symbiotic relationship with a bacteria called Nostoc, and in most species, the sporophyte grows continuously from the base.
