How to save mesh ply file

Question

I am new to python. I was given the following code that generates mesh from source data and plots tree meshes. I would like to save (x,y,z) of each tree mesh as a separate file using a plyfile.

for tid,tree in enumerate(forest):
    tree_arr = np.array(tree).reshape((-1,6))
    vertices = tree_arr[:,0:3].astype(float)
    radius = tree_arr[:,3].astype(float)
    parent_id = tree_arr[:,4].astype(int)
    section_id = tree_arr[:,5].astype(int)
    
    if np.max(vertices[:,2]) > 30:
    
        fig = plt.figure(figsize=[16,16])
        ax = fig.add_subplot(111, projection='3d')
        ax.set_box_aspect([np.ptp(vertices[:,0]), np.ptp(vertices[:,1]), np.ptp(vertices[:,2])])
        for i in range(1,parent_id.shape[0],1):
            j = parent_id[i]
            p0 = vertices[j]
            p1 = vertices[i]
            v = p1 - p0
            mag = norm(v)
            v = v / mag

            not_v = np.array([1, 0, 0])
            if (v == not_v).all():
                not_v = np.array([0, 1, 0])
            n1 = np.cross(v, not_v)
            n1 /= norm(n1)
            n2 = np.cross(v, n1)

            r0 = radius[j]
            r1 = radius[i]
            rv = np.array([r0, r1])[np.newaxis]

            t = np.linspace(0, mag, 2)
            theta = np.linspace(0, 2 * np.pi, 15)
            t, theta = np.meshgrid(t, theta)
            x,y,z = [p0[k] + v[k] * t + rv * np.sin(theta) * n1[k] + rv * np.cos(theta) * n2[k] for k in range(3)]

# I am trying to save PLY mesh files with following code. However, it only saves a few points for each tree instead of mesh.
            x = x.flatten()
            y = y.flatten()
            z = z.flatten()
            vertex= zip(*[x,y,z])
            vertex = list(vertex)
            dtype=[('x', 'f4'), ('y', 'f4'),('z', 'f4')]
            M = np.array(vertex, dtype)
            el = PlyElement.describe(M, 'vertex')
            name = f'modeled_tree{tree[5]}.ply'
            PlyData([el]).write(name)