Hi again.  I am in the US at the moment and Tala, her parents and I went hiking in a forest just out side of San Francisco called Muir Woods yesterday (Mon 5th July local time).  The area is reknowned for its large stands of trees referred to by their common name of costal red-woods (Sequoia sempervirens).  These trees are amongst the tallest alive and are one of only two remaining members of the genus sequoia.  Supposedly you can buy seeds for the other variety (Sequoiadendron giganteum) from a seller that appears to be in Victoria Australia (although it is often a little hard to tell where eBay sellers are from).

http://cgi.ebay.com.au/Giant-Redwood-Sequoia-Giganteum-50-Seeds-/150461709805

They are great trees which can grow up to 100m tall, 12m in diameter and can live for 3000 years (http://www.nps.gov/muwo/upload/unigrid-muwo.pdf).

Anyway amongst these trees in Muir Woods is a heap of Poison Oak.  I had always heard in movies how people have got “cases” of poison oak but I had never seen it.    In Australia we have stinging nettles and of course thorney brambles like blackberry but we don’t have either poison oak or poison ivy.   One of the rangers at Muir’s Wood showed me what poison oak looks like so we could see to avoid it.  You are advised not to leave the path in those woods so it is a very tame place to hike, but with that much poison oak around you probably wouldn’t want to hike through it any way.

I took some pictures of it.  It is a low growing plant, sort of like a ground cover but not that dense but grows from a creeping vine. 

This is a plant with some young growth on it.  There is a poem they tell kids to remind them how to recognise poison oak from normal plants saying “if it has leaves of three, let it be”.  You can see the slightly lobed leaves thich are in threes off the stem.

This plant has an older set of leaves to show what it looks like when they are more mature.

Lastly, this is a picture of the plants from the side to show how they are a short somewhat spindly plant with big leaves that look like oak leaves.

Poison oak, poison ivy and another plant called Sumac do not sting like stinging nettles as they do not have any thorns, rather they have a sap that contains an oil called urushiol.  As an oil Urushiol is absorbed into the skin and induces your immune cells, specifically T-lymphocytes to release immune mediating molecules causing severe inflammation.  Thus the reaction is not an allergic reaction but rather a hyperimmunity induced directly by the compound its self.  There are some people who do not respond to it at all, but many even if they do not develop a strong response at first, will exhibit a stronger response upon subsequent exposures due to the activation and proliferation of the appropriate T-cells after the first exposure.  So it is kind of an interesting plant with an interesting biochemistry.

Another interesting place we visited earlier in the day was Telegraph Hill in San Francisco.  It is best known for the 64m high Tower called Coit Tower, built specifically to beautify San Francisco at the bequest of wealthy socialite Lillie Hitchcock Coit.  I had heard from others who had been there though that there was a flock of parrots that lived on the hill.  A little investigation found that these were the so called “Wild Parrots of Telegraph Hill” as depicted in the 2005 documentary of the same name (http://www.imdb.com/title/tt0424565/; http://www.wildparrotsfilm.com/index.html).  The parrots themselves are South American cherry headed conures and are suppose to have arisen from escaped pets (http://www.wisegeek.com/what-are-the-wild-parrots-of-telegraph-hill.htm).  Sadly when Tala and I visited Telegraph Hill they were no where to be found, indeed a number of residents of San Francisco that we spoke to had never even heard of the parrots.  We will have to keep an eye out for the birds on future visits.