base_url() giving error

[eluser]bohara[/eluser]
This code in my layout file

Code:

<link rel="stylesheet" href="<?=base_url()?>css/blueprint/screen.css" type="text/css" media="screen, projection">

Gives this error when output

Code:

Call to undefined function: base_url() in <b>/Library/WebServer/Documents/ci/system/application/views/layout.php

It stops ouput right after the page title and I only see the error message when I view source.

Not sure what i am doing incorrectly here?