First you need to determine if it is Morus: look at other characteristics such as stipules, hairiness, venation and in the case of Morus does it have latex? No latex then it isn't Morus.

Here http://efloras.org/florataxon.aspx?flora_id=2&taxon_id=121220 is a description of the genus and a key with links to the species in China.

Might be a Convolvulus or maybe a Passiflora....

Nice picture.

Leaf looks like the leaf of Passiflora.

This is not Passiflora or Convolvulus. if you have flower photo. it may be help for the correct name identification. 

Thank you.

It was born spontaneously a few months ago. It has not flowered yet

nearest

Thank you.

I think is not Convolvulus althaeoides. Is not a climbing plant or plant that grows prostrated.

I think it could be related to Brachychiton acerifolius or similar species

No, as your plant is nevertheless a weed and I know how seedlings of Brachichiton look like. You have got some seed inside the soil, new soil?, that just grows if it was not you who put it into the pot.

The soil has many years and has not added again. I think the seed came from a bird excrement

good thinking.

As I still think it is a weedy plant: do you have got a book about plants growing in the wild in Spain? Maybe to seek your plant there.

e.g. a convolvulus not aways creeps and uses tandakles.

Yhank you!

Yes I have information on the Spanish wild plants, but does not match anything known. I think it's a non-native species, but unknown to me and to other botanical colleagues.

I agree with you that seems like a weedy plant

I'm still not convinced with what I found looking for plants with 5-lobed leaves.

Somebody thinks it might be a mulberry. And that is of course brought around by birds, and is growing fast. And if it does not get flowers like normal weedy plants... then it might be really one kind of a mulberry.....

Amazing!

My plant has exactly the same characteristics.

It seems that the most likely species is Morus alba:

http://s18.photobucket.com/user/saltcedar/media/P1010004.jpg.html

...or maybe Morus platanifolia

Thank you very much for your help.

I thought I could be some species of the genus Abelmoschus (Malvaceae), but I agree with the identification of S. Wenkart young plant Morus (Moraceae). The stems and leaf divisions are coincident with the latter.

Thank you. It was a pleasure to look for the right answer, together.

It is not Morus alba.

Without flowers, fruits and root even, it is simply not possible to identify the species.

I am still not sure about mulberry, as leaf lobes look a little too straight and narrow than what generally observed in Morus species (within their great variability)...but we should wait for the flowers. Let us know about it!

Certainly the leaves have lobes too straight and narrow for tipical Morus species.

I would wish a fast flowering just for resolving this enigma!

But pity: if it is really a Morus, we have to wait for the flowers until spring.....

Flowering is important to say the tentative name of the plant. Without flower, it is not praiseworthy to identify the name of the plant.

First you need to determine if it is Morus: look at other characteristics such as stipules, hairiness, venation and in the case of Morus does it have latex? No latex then it isn't Morus.

Here http://efloras.org/florataxon.aspx?flora_id=2&taxon_id=121220 is a description of the genus and a key with links to the species in China.

Hi Graeme Ure, thank you for the florataxon. Maybe I shall need it one day.

As if it is Morus, the simplest thing is to buy some silkworms and let them try some leaves.

It may be malvaceae member  if you have shows the photos it most useful for identification. 

It may be Passiflora  member

Thanks for your help.

I think it is Morus alba.

The plant has latex, and the stipules are like those of the genus Morus (see eg http://herbarium.bh.cornell.edu/imgs/robbin/r/Moraceae_Morus_alba_35129.html).

I attach pictures of these characters

Looks like wild okra. Observe for flowers and fruits.

Thanks, definitely Morus alba!

Is identical to the plant shown in the website: http://annieinaustin.blogspot.com.es/2010/10/fast-growing-5-lobed-mystery-plant.html, we suffer the same mystery!

It is a mulberry plant, leaf characters are matching with Mulberry (Morus); but this is  wild Okra Plant, that leaf contain serrate margin. Kindly observe the flower characters are very important.

Looks like okra, Abelmoschus sp. Better to check on flower and fruits.

So, we all wait for flowers and fruits.....

we have little doubt about the species, you have to check the floral charachters

Miguel, this plant is not Morus alba.  The linear lobes are not in the sequence of leaf forms found in that species.  You can see that sequence here:  http://www.washingtonpost.com/wp-srv/special/metro/urban-jungle/pages/120724.html

This linear lobed form is the juvenile leaf form within the Morus australis sequence.  In particular at its extreme, Morus australis can have simple tri-lobed leaves with only faintly-toothed leaves.  That has been historically referred to as Morus australis var. linearipartita, although now it is just considered part of the Morus australis complex.  You can find its description here:

 http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=200006380

http://www.plantphoto.cn/tu/1324979

Thank you.

Certainly looks Morus australis!

@ Miguel Angel Gómez-Serrano

It is difficult to identify a plant without reproductive structures. However, this plant is certainly not Morus australis.

Thanks!

@Arvind Singh, can you be more specific about your statement that this certainly cannot be the juvenile, fast growth form of leaves of Morus australis?

Yes, Dr. Arvind is correct, It is difficult to identify a plant without reproductive structures.

The image is very close name Morus inusitata H. Lév. and Morus bombycis Koidz. var. angustifolia Koidz., now all these names are synonym of Morus australis Poir. Many forms of Morus australis are there which differs in leaves shape and size, but the basic structure of flowers are same in all.

In China this form can be seen.

You need to see the flowers of the plant in addition to other parts in order to identify it easily.

Following......

The poster was asking for help because the flowering parts were not present; he already knows it is not an easy identification of an unknown. This is a mulberry (Morus). It may take 15 years before it flowers, and then it may only have male flowers. So to suggest wait 15 years to maybe measure the length of stigmas and fruit size to determine the species is not helpful either. He posted it here to engage those who have experience and direct knowledge of this plant and could help him overcome the constraints of identifying a newly arriving foreign species that does not appear in any of the local floras. It is a worldwide challenge. This taxon appeared in the United States relatively recently (~20 years ago) and still does not occur in any of the floras. In the past 5 years it is appearing frequently in online plant identification groups. Knowledgeable foresters and horticulturists are seeing it and being puzzled, they know Morus alba and its various leaf forms. The forms of that species as the name is customarily applied do not include this plant with linear lobes particularly in its juvenile and fast growing stages. In its extreme form it has simple linear lobes and is referred to in China as 'Chicken mulberry"

But more often it has leaves the appear like these:

or these

it may be a species of the genus Abelmoschus, having about fifteen species. But it will be good to observe it until flower for identification