Ailanthus. This tree was one of the very first whose pollen was known to cause respiratory discomfort. Unfortunately the observation was first made by botanists and was not generally known until years later. In 1905 the botanist Julia Rogers, speaking of the ailanthus states, “The staminate trees had a rank odour and the pollen annoyed people with catarrh.” Had this observation been noted by investigators of the hay fever problem it might have advanced the work on tree pollens in hay fever by about fifteen years.
The tree is a native of China, introduced in 182o into New York City where it now grows abundantly. The pollens are shed during June over a period of one week. The tree can be easily recognized by its fern like leaves, rapidly growing one inch-diameter shoots, and characteristic horseshoe-shaped bud scar.
Alder. The alders are, in general, short, water loving trees, found in the United States; Canada and Europe. In the United States the species of alder that cause hay fever occur in New York, Pennsylvania and the northeastern states in general. In the south they are spread along river bottoms from Florida to Texas. Other species are spread through Nebraska, Idaho and California.
The alders pollinate early in March before the leaves appear. They have oval shaped leaves, vase-shaped leaf buds, and drooping catkins 2 to 3 inches long. A recent report by Dr. Stroh indicates the alder as. an important cause of hay fever in Seattle, Washington, during the month of March.
Ash. There are as many as 5o species of ash trees. Only a few have been reported as responsible for hay fever. Effects from species of ash tree pollens have been reported in New York. These trees assume greater importance in Colorado, Utah, New Mexico, Texas, Arizona and Missouri.
The trees are generally tall, growing to a height of 70 feet or more. The twigs have a characteristic four-angled appearance. Pollination occurs in April and May.
Birch. The birch trees grow chiefly in the eastern half of the United States. The yellow, black, gray, and sweet birch are important hay fever-causing trees. They are known to shed an abundant amount of pollen. Although each species pollinates within a week, Wodehouse’s study in New York City showed a large amount of birch pollen to be present in the air from the beginning to the end of May. This was due to the combined effect of the various species of birch occurring in that vicinity.
The trees may be recognized by their characteristically smooth bark and conspicuous horizontal slits. The stamens are borne in long catkins which shed the pollens before the leaves appear.
Elm. Both the white elm and the slippery elm, are important causes of hay fever. They are spread across the continent from east to west but are most abundant in the New England states. New Haven, the home of Yale University, by reason of its many elms, has been called the “Elm City.”
Severe cases of elm hay fever have been recorded but they are of short duration. Generally these trees pollinate during the month of April, much before the leaves appear. However, it has been reported that the scrub elm which pollinates later in Dallas, Texas, causes trouble and is frequently confused with the autumn ragweed symptoms. The leaves of the elm can be distinguished by their doubly serrate margins and sides of unequal size.
Hack berry. The hackberry trees are known to cause hay fever in the Mississippi Valley and Texas but are harmless in the northeastern states. Large concentrations of their pollen have been reported near Nashville, Tennessee. The hackberries are closely related to the elms but unlike them they pollinate after the leaves appear. The pollination period occurs during April and May.
Hickory. Twelve known species of hickory occur in North America. They are restricted in the United States to the states east of the Rocky Mountains. All the species are found in Arkansas. The six species that grow in New York produce a large amount of pollen from the middle of May to mid-June. The stamens are borne in large catkins which distribute the pollens when the leaves are half grown.
Maple. Although spread throughout the eastern half of the United States the pollens of maple are not very active.
A particular species, the box-elder, causes hay fever effects in Colorado, southern Texas, Missouri and Utah. The box-elder is wind pollinated while the other species of maple employ insects as a means of pollination.
Mulberry. The mulberries are distributed in an area of the United States bounded by Southern New York, Florida, Iowa, and Texas. The paper mulberry has been re-ported as a prevalent cause of hay fever in Washington, D. C. Because of the fact that the silkworm feeds on the leaves of the white mulberry, it is reputed to be the most important tree ever cultivated. Pollination occurs in these trees during May and early June.
Mountain Cedar. The Mountain Cedar is especially interesting as a hay fever plant because it produces its effects in winter rather than spring. It does not shed its leaves and is used as an evergreen Christmas tree in Texas where it abounds. Thus the pollens are often brought into the home. The tree is confined to Texas, Arizona and New Mexico.
Mesquite. Additional causes of hay fever in Texas are the mesquites. They grow as small shrubs, with deep underground roots, in dry regions and also as trees of respect-able size in areas of adequate moisture. They shed their pollens in May, June and July.
Oak. The oaks with their abundant production of pollen are considered by many to be the chief offender as a cause of hay fever. But in his experiments in New York, Wodehouse pointed out that despite the fact that oak pollens outnumbered all other pollens, there was relatively little hay fever caused by them. There are about 22o species of oak, and one or more kinds are found in every state of the United States. They pollinate chiefly during May when their uniquely shaped leaves are about one-third grown.
Poplar. Of this group the western cottonwoods are regarded as a frequent cause of hay fever in New Mexico and Arizona. They are also reported to be responsible for many cases in California, Texas, Utah, South Dakota and Missouri. They pollinate during April before the leaves appear. The species of poplar that grow in the East seem to cause little or no hay fever.
Sycamore. Like the cottonwoods, the species of sycamore that grows in the west is the culprit responsible for hay fever. Although widely spread throughout the eastern half of the United States the species in this region seem quite harmless. The tree is easily recognized by its broad flat leaf and seed balls or buttons that swing from the branches during the winter. Pollen in abundant quantities is shed by the sycamores during April and May.
Walnut. The pollens of the walnut tree are frequently shown to cause reactions in allergic patients. However, these trees are rather sparse in the East. In California where the walnut is cultivated on a large scale it is re-ported as a hay fever factor. It is also considered to be responsible for some hay fever in Arizona, Oregon, Colorado, and New Mexico, during May and June when it pollinates. The walnuts are closely related to the hickories and pecans which are also known to cause hay fever symptoms.