Diatom valves differ between one species and another - each species is unique and can be recognized on the basis of its valves alone (as yet, there are no known examples of cryptic speciation in diatoms, where species cannot be distinguished on the basis of morphology). However, no two individuals within a species are exactly the same; there is a certain amount of morphological variability within species or even within infraspecific taxa (varieties, subspecies, forms) which can be due to developmental "noise" and/or interactions between the genome and its environment.   Furthermore, the diatom life-cycle comprises an asexual phase that involves gradual reduction in size, and a sexual phase that restores size to the maximum for the species. In the asexual phase each cell divides into two copies of itself, one of which is the same size as the mother cell, the other of which is slightly smaller. The reason for this is that the new valves and girdles form inside the mother cell, and one of the mother cell's valves becomes the larger valve of each daughter cell. So from a single cell there arises a clone of genetically identical individuals which are progressively smaller with each succeeding asexual generation. This part of the life-cycle continues for anything from one to several years, but is of course not sustainable indefinitely. Eventually, through a combination of size and environment, the diatoms are triggered to enter the other phase of their life-cycle and become sexual - they form gametes that fuse to become a specialized cell which swells to the maximum size for the species. This so-called auxospore then divides asexually and initiates the asexual phase of the life-cycle again. Thus, absolute size changes dramatically within the life-cycle (Fig. 6).  Shape changes may also occur during size reduction. Typically, the length of diatoms reduces faster per cell division than width over most of the size-reduction sequence. This results in a change of aspect ratio, diatoms becoming relatively more isodiametric as size-reduction progresses. Also, complications in the outline tend to even out. However, the ornamentation of the valve surface tends not to change very much with size-reduction: pattern periodicity stays more or less the same, which means that smaller valves contain fewer pattern units (for example striae) than do bigger valves of the same species - the striae tend not to get much closer together.