why do some herbaceous plants have hydathodes?under what conditionsdo they help the plants

Dear Student,
 
The process by which plants loose water in liquid form is called Guttation. Guttation takes place only under certain conditions of  high humidity in the atmosphere and plants with high amount of  water in the soil.
 
Hydathodes are also known as water stomata or open pore. These are microscopic pores which are always open through which guttation takes place. Hydathodes are mostly found in aquatic plants and in some herbaceous plants. 
 
Hydathodes are mainly located on apex and serrated edges in the margins and surfaces of leaves which are responsible for secreting water in liquid form (guttation). 
 
Water is excluded through the hydathodes by theguttation process under conditions of water uptake and limited transpiration, such as high humidity in the dark.
 

Regards,