diff --git a/skimage/morphology/binary.py b/skimage/morphology/binary.py
index 636717af..d5e0d92b 100644
--- a/skimage/morphology/binary.py
+++ b/skimage/morphology/binary.py
@@ -18,8 +18,9 @@ def binary_erosion(image, selem=None, out=None):
     ----------
     image : ndarray
         Binary input image.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
+        If None, use cross-shaped structuring element (connectivity=1).
     out : ndarray of bool
         The array to store the result of the morphology. If None is
         passed, a new array will be allocated.
@@ -66,9 +67,10 @@ def binary_dilation(image, selem=None, out=None):
 
     image : ndarray
         Binary input image.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray of bool
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray of bool, optional
         The array to store the result of the morphology. If None, is
         passed, a new array will be allocated.
 
@@ -113,9 +115,10 @@ def binary_opening(image, selem=None, out=None):
     ----------
     image : ndarray
         Binary input image.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray of bool
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray of bool, optional
         The array to store the result of the morphology. If None
         is passed, a new array will be allocated.
 
@@ -145,9 +148,10 @@ def binary_closing(image, selem=None, out=None):
     ----------
     image : ndarray
         Binary input image.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray of bool
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray of bool, optional
         The array to store the result of the morphology. If None,
         is passed, a new array will be allocated.
 
diff --git a/skimage/morphology/grey.py b/skimage/morphology/grey.py
index 7fb1c8c9..e50e1460 100644
--- a/skimage/morphology/grey.py
+++ b/skimage/morphology/grey.py
@@ -22,12 +22,13 @@ def erosion(image, selem=None, out=None, shift_x=False, shift_y=False):
     ----------
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarrays
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarrays, optional
         The array to store the result of the morphology. If None is
         passed, a new array will be allocated.
-    shift_x, shift_y : bool
+    shift_x, shift_y : bool, optional
         shift structuring element about center point. This only affects
         eccentric structuring elements (i.e. selem with even numbered sides).
 
@@ -79,12 +80,13 @@ def dilation(image, selem=None, out=None, shift_x=False, shift_y=False):
 
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray, optional
         The array to store the result of the morphology. If None, is
         passed, a new array will be allocated.
-    shift_x, shift_y : bool
+    shift_x, shift_y : bool, optional
         shift structuring element about center point. This only affects
         eccentric structuring elements (i.e. selem with even numbered sides).
 
@@ -136,9 +138,10 @@ def opening(image, selem=None, out=None):
     ----------
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray, optional
         The array to store the result of the morphology. If None
         is passed, a new array will be allocated.
 
@@ -191,9 +194,10 @@ def closing(image, selem=None, out=None):
     ----------
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray, optional
         The array to store the result of the morphology. If None,
         is passed, a new array will be allocated.
 
@@ -245,9 +249,10 @@ def white_tophat(image, selem=None, out=None):
     ----------
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
-    out : ndarray
+        If None, use cross-shaped structuring element (connectivity=1).
+    out : ndarray, optional
         The array to store the result of the morphology. If None
         is passed, a new array will be allocated.
 
@@ -299,8 +304,9 @@ def black_tophat(image, selem=None, out=None):
     ----------
     image : ndarray
         Image array.
-    selem : ndarray
+    selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
+        If None, use cross-shaped structuring element (connectivity=1).
     out : ndarray
         The array to store the result of the morphology. If None
         is passed, a new array will be allocated.