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All About Trees (Winter Course)

January 16 @ 6:30 pm - February 20 @ 7:30 pm

WINTER COURSE: BOTANY SERIES – Free course with Professor Karen Snetselaar at the Philadelphia Horticultural Society


All About Trees with Professor Karen Snetselaar

6 Tuesdays from January 16 – February 20, 6:30 – 7:30 PM

Location: This course meets at the Pennsylvania Horticultural Society, located at 100 N. 20th Street in Philadelphia.

This course requires pre-registration. Separate registration is required for each student. Sign up here or call 215-763-6529 x14 for assistance.

Course Description

This course will explore several fundamental properties of trees and their importance in our world. Each week the class will consider what science can tell us about one of these properties and apply that knowledge to answer popular questions such as: How are trees able to grow so large? Do they really communicate with one another underground? Why are trees so vital to ameliorating storm water problems in urban areas?

Course Schedule

1. Tuesday, January 16, 2024 – How Trees Become Trees: Secondary Growth All plant growth originates with groups of cells that are capable of dividing over and over. These groups of cells at the tips of plant stems allow them to get taller. This is called primary growth. Only trees (and shrubs) have stems that can get bigger around, in a process called secondary growth. This week will focus on this defining quality of trees. We will see how it leads to the production of annual growth rings and how the massive appearance of tree trunks produced by secondary growth hides their vulnerability.

2. Tuesday, January 23, 2024 – Leaves: Photosynthesis Factories The leaves are where nearly all the “stuff” that goes to make up a tree is produced from simple ingredients—water, carbon dioxide, and relatively small amounts of a few other molecules. The driving force for this production is energy from the sun that trees (like other plants) use to “fix” inert carbon dioxide gas into energy-containing sugars. All tree leaves use basically this same process. So why do leaves come in so many shapes and sizes?

3. Tuesday, January 30, 2024 – Trees Underground: Taking up Minerals and Water Most plants have as much living material underground as they do above the ground, and trees are not exceptions to this rule. Almost all the water and minerals that trees need to grow are taken in by their root systems. Trees develop complex relationships with underground microbes that enhance their abilities to take up these nutrients. This week will examine evidence for recent popular ideas about trees “communicating” with each other underground.

4. Tuesday, February 6, 2024 – Transport in Trees Nearly all the water for that giant redwood or spreading oak comes from the soil—how does the tree get the water up to the leaves where it is needed for photosynthesis? Actually, the water problem is more about how much water is lost through pores in leaves that they have to open to let the carbon dioxide in. A single tree can move over 100 gallons of water a day from the soil to the air. This is just one of the ways that trees help reduce negative effects of flooding from storms; we will talk about others too.

5. Tuesday, February 13, 2024 — Seeing the Trees in the Forest By definition, forests habitats are primarily defined by the species of trees found there. This week will focus on ways that trees influence other living things in different forest and woodland types.

6. Tuesday, February 20, 2024 – City Trees While we tend to think of forests when we think about trees, the truth is that trees have evolved for many different habitats. Some trees are adapted to meet the challenges of urban life, such as compact soil, reduced root area, and periodic droughts. But even many of these adaptable trees are struggling with the combined effects of climate change and increasing urbanization. What can be done to help promote tree growth in our towns and cities?

February 27 is reserved for a make-up class, if needed.

Suggested Reading

A list of reference materials will be provided.

About the Professor

Dr. Karen Snetselaar is Professor Emeritus of Biology at Saint Joseph’s University. She has been a member of the faculty of the Wagner Free Institute of Science since 2000. Her research focuses on fungi and plant disease. She is also interested in urban ecology and in promoting public understanding of science, especially science education outreach into K-12 schools. For 20 years, she led the Wagner’s partnership with Saint Joseph’s University, GeoKids LINKS, which brings intensive science learning into North Philadelphia schools.


What is the cost?

Wagner courses are free; donations are encouraged!

Are there minimum age requirements to enter the course?

The adult lecture courses are offered on an introductory college level. They are open to anyone seeking an introduction to various areas of the sciences as well as those familiar with the subjects who wish to broaden and update their knowledge. The courses also provide an excellent opportunity for motivated junior high and high school students to supplement their current science courses and to help prepare them for further study in these fields at a college level. The courses are appropriate as well for teachers seeking to expand their knowledge.

What are my transportation/parking options for getting to and from the course?

The Pennsylvania Horticultural Society is located at the northwest corner of 20th and Arch Streets in Center City. It is convenient to multiple public transportation routes. The closest trolley stop is at 22nd and Market Streets. The location is also accessible via multiple bus routes and regional rail lines. There is on-street parking on the surrounding blocks, and multiple paid parking options, including a lot at 19th and Arch Streets.

Is the building wheelchair accessible?


How can I contact the organizer with any questions?

Click the “Contact the Organizer” link, email communications@wagnerfreeinstitute.org, or call 215-763-6529 x14.

Images from top to bottom: Mangrove tree roots in jungle via CanvaPro. Tree roots resisting lateral forces, 1911 via Popular Science Monthly Volume 79, WikiCommons.


January 16 @ 6:30 pm
February 20 @ 7:30 pm


Pennsylvania Horticultural Society
100 North 20th Street
Philadelphia, PA 19103 United States