The iris plant or shoubu (菖蒲) is a popular motif in Japanese art. The word for iris is shoubu but when the same word is written with different kanji it can mean victory (勝負) or militarism or martial spirit (尚武). The Japanese like this kind of play on words called goro awase.
This tsuba above was created by Choshu Hagi no ju Inoue Michitaka saku 長州萩住井上通高作 in the mid 18th century.
The iris plant known in Japan as kakitsubata is supposed to represent strength and health and is said to ward off evil spirits so it was a often used motif in samurai accouterments like tsubas and armor. Kakitsubata is also a name of a famous Noh play based on a passage from the Tale of Ise. The plant itself is a nice symbol with a lot of hidden meaning other than the clever word-play.
From the photo of an ikebana arrangement, we can see that the leaves stand up and are long, straight and pointed which look like swords. Within one plant it looks like many swords standing up, but with many rows of plants (see painting above) it looks like an army staging before a big battle hence this idea of militarism. Another nice symbolism is that the flower or true inner beauty only comes out once the leaves have grown tall which gives us this idea that growth and experience can bring out one's true inner beauty.