EPITHEM-HYDATHODES 495 



their power of adjustment to a large extent. As a matter of fact, they 

 usually become stationary at an early stage of development {Fuchsia), 

 while not infrequently they are from the first incapable of movement. 

 In accordance with this relative immobility, the cuticular ridges, which 

 form so characteristic a feature of typical guard-cells especially when 

 seen in transverse section are reduced {Tropaeolum, Fig. 199 d), or 

 entirely absent. The guard-cells are sometimes short-lived {Tropaeolum, 

 Colocasia, Aconitum), and occasionally disappear altogether {Hippuris, 

 Callitriche). In certain cases (Akoideae, Pa/paver, Tropaeolum) the 

 water-stomata are remarkable on account of their large size. 



According to Tswett, the marginal epithem-hydathodes on the 

 leaves of Lobeliaceae are furnished with water-stomata of an unusual 

 type. The wide pore of each stoma is divided in two by a cutinised 

 septum, which evidently represents the middle lamella of the partition 

 wall between the two guard-cells. The cuticle also remains imperforate, 

 so that the pore is never open. The author has confirmed Tswett's 

 observations in the case of Lobelia syphilitica. Tswett states that the 

 permanent closure of these water-stomata does not prevent liquid 

 water from exuding under suitable conditions. The cuticle must 

 therefore be pervious to water at these points. What the advantages 

 of this peculiar type of structure may be, is quite unknown. 



The greatest diversity prevails with regard to the number of water- 

 stomata that are associated with a single hydathode. In Fuchsia, Aconi- 

 tum, Delphinium, Primula, etc., according to De Bary, the tip of each 

 leaf-tooth is provided with a single large gaping water-pore. Groups 

 comprising from three to six pores occur in Ulmus campestris, Crataegus 

 coccinea, Hellcborus niger, etc. In certain Umbelliferae and Compositae, 

 finally, and in Potentilla, Alchemilla, Geum, etc., numerous water-stomata 

 are crowded together within a small area. The disc-shaped epithem- 

 hydathodes which occur in spp. of Ficus, Conocephalus and other 

 Mohaceae, are likewise provided with a large number of small water- 

 stomata. In hydathodes of this type the epithem is usually shut off 

 from the adjoining green mesophyll by a parenchymatous bundle- 

 sheath, the cells of which frequently have their inner walls cutinised. 



Some very simple hydathodes consist of water-pores without any 

 associated epithem. Here the free ends of the terminal tracheides border 

 immediately upon the intercellular spaces of the mesophyll, which in 

 their turn communicate with the hypostomatic chambers of the water- 

 pores. The leaf of Vicia sepium bears a solitary hydathode of this 

 type, with from five to eight water-pores, at the tip of each pinna. In 

 certain Grasses, again {Secale ccrealc, Triticum vulgare, Zea Mays), the 

 author has observed epithem-less hydathodes at the tips of the coty- 

 ledonary sheaths and primary leaves ; in these cases the water-stomata 



