我正在努力将Phylo树保存到文件中。它显示在Jupyter Notebook中，但文件为空白。

我浏览了很多文档，也浏览了很多Google文档...如何获取要打印到文件的树？

# Read in alignment
aln = AlignIO.read("v4.fna","fasta")

# Calculate the distance matrix
calculator = DistanceCalculator('identity')
dm = calculator.get_distance(aln)

# Visualize neighbor joined tree
constructor = DistanceTreeConstructor()
tree = constructor.nj(dm)
fig = plt.figure(figsize=(10,20),dpi=100)
axes = fig.add_subplot(1,1,1)
Phylo.draw(tree,axes=axes,label_func=get_label)
plt.show()
plt.savefig('v4.png')