We also supply 3 optional black plastic sources, these can be used as drip trays for small pot plants, however we believe their main attraction is when you are doing the vertical sprouts, like wheatgrass or snow peas, where you use this as the reservoir rather than the one that comes with the pot, the reason being is twofold. Firstly it gives you a larger reservoir, which can be important if the plants are in full sun in summer, and secondly being larger than the plant pot itself you can easily see at a glance whether you need to add more water, making it very easy to fill.