Difference between initializations stringstream.str( a_value ) and stringstream << a_value

Consider:

std::string        s_a, s_b;

std::stringstream  ss_1, ss_2;

// at this stage:
//     ss_1 and ss_2 have been used and are now in some strange state
//     s_a and s_b contain non-white space words

ss_1.str( std::string() );
ss_1.clear();

ss_1 << s_a;
ss_1 << s_b;

// ss_1.str().c_str() is now the concatenation of s_a and s_b, 
//                    <strike>with</strike> without space between them

ss_2.str( s_a );
ss_2.clear();

// ss_2.str().c_str() is now s_a

ss_2 << s_b;  // line ***

// ss_2.str().c_str() the value of s_a is over-written by s_b 
// 
// Replacing line *** above with "ss_2 << ss_2.str() << " " << s_b;"
//                    results in ss_2 having the same content as ss_1.

Questions:

1. What is the difference between stringstream.str( a_value ); and stringstream << a_value; and, specifically, why does the first not allow concatenation via << but the second does?

2. Why did ss_1 automatically get white-space between s_a and s_b, but do we need to explicitly add white space in the line that could replace line ***: ss_2 << ss_2.str() << " " << s_b;?