I was of the same opinoin until I loticed how long some of the stems were. Then, in other images (which I will try to link one from my goofed up article) I saw that the stems sometimes came from the side. There seem to be more like this than straight up ones. These could not be hoodoos, because the stem is exposed well away from the sphere, and in different directions on the same rock. Some of these even have the sphere still supported above the surface.
As to what they are, from the beginning I noted features of the spatial and size distribution of the spheres arguing against them being concretions. As far as I am concerned, the concretion theory of Nasa is deader than dead, and has been shot down by the evidence from numerous photos on all scales. To whit, the rock layers do not pass through the spheres. Hundreds have been imaged that were cracked or worn down, and the only internal features are small rings and spots without much order. The spheres are usually several times larger in diameter than the layers, so we should frequently see layering going through them if they were concretions. Nasa's "evidence", a single sphere with some grooves in it was laughable; there are a number of the better preserved spheres that show grooves in different, random directions. Here, they just found one where the grooves just happened to align with the layers, and claimed this showed how the layers penetrate the sphere. Sorry, it doesn't show that at all. Those layers could have formed concurently with the sphere, and cauysed the grooves. This seems unlikely, because right close by this berry was another with the grooves oriented at an angle to the layers. Talk about your selected evidence. This has started to look like a theme; select the evidence to support your theory, and ignore everything else. Investigate any alternative hypothesis? Bah humbug. We KNOW we're right, we're Nasa gurus. At this point in time, I knew something was up there at Nasa that I didn't like. I won't say that its a conspiracy, because it could just be incompetance. Either way, whats up?
Another glaring problem with the Nasa concretion theory is the lack of fused spheres. Concretions usually cluster together. In Meridiana, they have gone a long way and it still looks like only about 1% of the spheres are malformed, and even less that are fused. More tellingly, larger clusters or layered groups dont seem to exist at all. Small zones of isolated perfect spherical concretions do exist, but a whole region of them? This is extremely unlikely. Again, Nasa selectively homed in on one of these fused examples, the triple berry, to claim it as evidence that these are concretions. If that was the only picture that I saw, I would agree with them.
The spheres could be some type of precipitation, like a cave pearl, only casually related to the segmented filamentous stems, as I discussed above. However, it seems that there is a general vertical orientation to the stems which are seen attached to spheres. This would argue that they were supporting structures growing up from the bottom sediments as one would expect for a stalk structure. This sphere/stem association is critical to determine the origin of the spheres, which Nasa has just brushed aside as a "solved" puzzle. They need to pull their head out of the sand, or what ever else it got stuck in, and start using the micro camera to take some shots of these spheres with stems. They have passed up dozens of Opportunities to do so.

This is the picture with the spheres with stems sort of flopped over on the lower left, and more upright between the tread tracks. I counted 6 with stems visible on this lower rock. here
In case that didnt work, the url: