300 



MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



to tiny pointed scales about ^ mm. in length; two of them are situated at the 

 mouth of each cavity, and are inflected towards one another across it. In Bolbo- 

 phyllum Odoardi, each of the small tubers bears only one small green leaf, which 

 is about 1^ mm. long and 1 mm. broad, and is placed close to the opening of the 

 chamber (see fig. 70*'^'^). Stomata are found exclusively in the interior of the 

 hollow tubers. Water cannot enter through the narrow mouth into the air-containing 

 chamber, and even when, in the rainy season, the whole of the mossy carpet, in 

 which these smallest of all orchids are interwoven, is saturated with water, their 

 transpiration continues unhindered, provided that the other conditions on which it 

 depends are fulfilled. It is obvious that these structures which prevent moisture 

 reaching the stomata during the wet season of the year can take on another function 



^^ 



Fig. 70.— Orchids whose Stomata lie in Hollow Tubercles. 



^ Bolbophylluin minutissimum. 2 j\^ tuber seen from above ; x8. * Vertical section through this tuber; xl6. * Bolbophyllti 

 Oduardi. s A tuber; x6. « Longitudinal section through this tuber ; x6. 



in a succeeding dry period, which may follow immediately; but this must be 

 spoken of again later. 



The occurrence of "rolled leaves", which are observed in so many plants of widely 

 different affinity, is also connected with the keeping of water from the stomata. 

 The rolled leaf is always undivided, of small area, generally linear, but sometimes 

 ovate-linear, elliptical, or even circular in outline; always stiflT, and usually ever- 

 green, and therefore living through two or three periods of vegetation. Its edges 

 are bent down and more or less rolled back, even whilst still hidden in the bud. 

 In consequence of this, the lower side which faces the soil is hollowed to a greater 

 or less extent, while the upper side, turned skyward, is arched. Frequently the 

 leaf is rolled so as to inclose an actual chamber, which only communicates with the 

 outer world by a very narrow fissure, as is the case, for example, in the Crowberry 

 (Evipetrum). The rolled-back margins of the leaves in this plant almost touch 

 one another, and the epidermis of the lower side of the leaf forms the actual 

 lining of the cavity which resulted from the rolling of the leaf (see fig. 71 ^). 



