Like most social media platforms, Google+ gives you the ability to leave comments on posts. Comments are able to be edited after posting and contain links and +Tags the same as a post. You cannot attach photos or videos to comments. To leave a comment on a post, just click the Comments Text link under the post and type your comment. You cannot attach pictures or videos within a comment, but you can include links by starting with http://.
Each post is limited to receiving 500 comments. If you write a hot piece of content that gets that many comments, my suggestion is to keep the fire going by resharing your own post and inviting people to continue the conversation there.