Comment Gravatar Does Not Show!

can someone help the boxe where the visitors are supose to comment i not there! and the gravatar either! plz help!

The error:

<img border="0" style="padding:1px; float:left; margin:0px 10px 5px 0px; border:1px solid #ccc;" alt="avatar" src="
Fatal error: Call to undefined function gravatar() in /home/de3r7/public_html/wp-content/themes/Simplism/comments.php on line 18

The comment.php code ( 5 lines before and after line 18):

<?php foreach ($comments as $comment) : ?>

  <li class="<?=$oddcomment;?>">
   <img border="0" style="padding:1px; float:left; margin:0px 10px 5px 0px; border:1px solid #ccc;" alt="avatar" src="<?php gravatar("R", 48, get_option('home')."/wp-content/themes/Simplism/images/avatar.gif"); ?>" />

   <a name="comment-<?php comment_ID() ?>"></a><cite><?php comment_author_link() ?></cite> Says:<br />

   <div class="comment-inside">
   <small class="commentmetadata"><a href="#comment-<?php comment_ID() ?>" title=""><?php comment_date('F jS, Y') ?> at <?php comment_time() ?></a> <?php edit_comment_link('e','',''); ?></small>