How to call a child components method?

Hello, I have been having some trouble calling a child components method using refs as this example suggests:

Here is how I implemented the example into my code:

import React, { Component } from 'react';
import Fractal from './Fractal.js';
import Share from './Share';
import Helmet from 'react-helmet';

// App component - represents the whole app

export default class App extends Component {
	constructor(props) {
                this.appsenddown = this.appsenddown.bind(this);
	appsenddown() {
	render() {		
    	        return (
      		                <div className="container">
				                <Fractal onRef={ref => (this.child = ref)}/>
				                <Share triggerParentUpdate = {this.appsenddown}/>

The error message I get in my browser is:
TypeError: this.refs.child is undefined [App.js:14:2]

try it like this…

	appsenddown() {