为什么即使结构位于同一程序包中，数据存储区也不存储小写的结构属性？

package test

import (
    "net/http"
    "google.golang.org/appengine" 
    "google.golang.org/appengine/datastore" 

)

func init() {
    http.HandleFunc("/", handler) 
}


type myStruct struct {
    test string
}

func handler(w http.ResponseWriter, r *http.Request) {
    var ctx = appengine.NewContext(r)   

    datastore.Put(ctx, datastore.NewKey(ctx, "myStruct", "mykey", 0, nil), &myStruct{ test: "foo" } )

}

The above code will store the entity and its key name in the Datastore, but it does not store the property value "foo" unless I capitalize the property name to "Test". I thought upper case is not necessary when a variable is in the same package? I'm using the local development server.