1. Let $\left\{a_{n}\right\}$ be an arithmetic sequence with common difference $d \neq 0$, and the sum of the first $n$ terms be $S_{n}$. Then the sequence $\left\{S_{n}\right\}$ is increasing if and only if $\qquad$.
$$
\text { I. 1. } d>0 \text { and } a_{2}>0 \text {. }
$$

Obviously, $\left\{S_{n}\right\}$ is an increasing sequence
$$
\begin{array}{l}
\Leftrightarrow S_{n}0 \\
\Leftrightarrow d n+a_{2}>0 \\
\Leftrightarrow d>0 \text { and } a_{2}>0 .
\end{array}
$$