ii8 



LECTURE VIII. 



FIG. 117.— Transverse section of the leaf of Püuis 

 Pinaster (X 800). s guard-cells of a stoma (/), under 

 which is the air-cavity /; 7' entrance, surrounded by the 

 epidermis cells, the cuticle (r) of which is very thick ; 

 a the middle lamella, and t the thickening layers of the 

 sclerenchyma cells ; g cells of the leaf parenchym.i, with 

 the contents contracted. 



vapour can always be provided for by a corresponding increase in the number 

 of these fine openings, and by the plant at the same time possessing the power 

 of enlarging or diminishing the width of the stomata according to requirement. 



The extreme narrowness of these openings, 

 even at the maximum, is shown by the 

 statements of Mohl, according to which the 

 very large stomata of various species of Lilium 

 may be widened up to -^l-^ mm. at most, and 

 those of Indian corn {Zea Mays) up to yig mm. 

 at most ; while Unger gives the size of the open 

 fissures of Agapanihus umbellaliis as 0-000047 

 sq. mm. This extraordinary minuteness is 

 compensated by the large number of the 

 stomata. On the foHage leaves, they amount 

 per sq. mm. in the most frequent cases to 

 40-100, very often 100-300, occasionally even 

 600-700 and more; so that the epidermis 

 of a foliage leaf of any considerable size is 

 penetrated by many millions of these fine 

 openings. — In accordance with the object of 

 the stomata above indicated, they are funda- 

 mentally nothing but the mouths of the inter- 

 cellular spaces of the internal tissue which 

 are developed pre-eminently in the organs 

 rich in chlorophyll ; and the stomata are 

 also again only intercellular spaces between 

 peculiarly formed cells of the epidermis, the 

 so-called guard-cells. Every stoma, where 

 it penetrates the epidermis, is surrounded by 

 two peculiarly formed 'cells, the guard-cells, 

 the partition-wall of w'hich, originally simple, 

 has become split, and so formed the inter- 

 cellular space or pore of the stoma. The 

 figures here annexed show the condition of 

 things sufficiently for our purpose. If we 

 strip off the epidermis from a foliage-leaf, 

 and spread it out beneath the microscope, 

 the peculiarly formed pairs of cells which have 

 proceeded from one mother-cell, and have 

 produced the proper stoma by the splittmg of 

 their partition-wall, and thus surround it, are 

 easily recognised. Very often, especially in the 

 highly organised Phanerogams, another group 

 of peculiarly placed and formed epidermis cells are found around -the pair of guard- 

 cells ; these have arisen, before or after the development of the proper guard-cells, 

 from neighbouring epidermis cells by the corresponding formation of partition-walls, 



Fig. 118, young, and (FIG. 119) fully developed sto 

 mata of the leaf of the Hyacinth, s stoma in transverse 

 section ; t air-cavity; e, e neighbouring cells of the epi 

 dermis ; /, / parenchyma of the leaf. 



