64 THE STOMATA OF PLANTS. 



stomata on the upper surface ; 6 had fewer stomata on the upper than en the under 

 surface ; and 5 had an equal number on both surfaces, leaving only two instances in 

 which the number was greater on the upper than on the under surface of the leaf. 



The number and position of the stomata must have an immediate reference to their 

 function. It is commonly understood, as has already been intimated, that the function 

 is that of admitting air and moisture to promote the digestion of the crude sap 

 which had been brought to the leaves, and that for this purpose they are endowed with 

 the faculty of opening and closing according to the momentary requirements of the 

 plant. This will explain the necessity for their conformation. As to their position, 

 that seems to be due to several causes. First, that by being placed on the under surface 

 they are shaded from the direct action of the sun's rays, and are thus permitted to 

 carry on their functions without being impeded by too great a degree of evaporation. 

 Secondly, they are also more sheltered from the injurious deposition of dust. Thirdly, the 

 exhalation of moisture from the ground is in the form of vapour, which, from its specific 

 gravity, rises, and thus reaches and enters the under surface more certainly than the 

 upper surface. It is not presunied that in any case water enters the stomates as such, 

 but only in the state of vapour ; for although plants are refreshed after a shower, it does 

 not follow that the rain was bodily introduced within them ; and it seems inconceivable 

 that bodies of so minute a size should at the same time be fitted for the admission of 

 gases, and of fluids of such density as water. 



There are those, however, who maintain that such is not the function of the 

 stomata, but that they arc in the nature of glands. Link says that he cannot 2nd a 

 distinct connexion between the stomata and the subjacent cavities in the cellular 

 tissue of the leaves. Moreover, he cannot understand how organs of so distinct a 

 structure should only lead to mere cavities in the cellular structure; and the obstructing 

 and covering matters which they produce have always led him to consider them as 

 organs of secretion. Brown also affirms that they are rather of the nature of glands ; 

 but there cannot be a doubt that in the vast majority of instances this view is incorrccv,. 

 It is true that in a few instances the stomata are modified both in figure and in function 

 to perform the office of glands. Such is the case in the Dionsea Muscipula, or Venus' 

 fly-trap (Fig. 1), in which the stomata are reduced each to a pair of parallel green 

 cells, which are placed upon the surface of the leaf, and secrete a tenacious mucus ; but 

 such are exceptional cases. 



It would be interesting if we could determine with certainty the precise mode in 

 which these beautiful organs are formed; but such seems hitherto to have been a hopeless 

 task. Mohl sought to determine it by examining the different parts of a growing 

 hyacinth, in the expectation that the parts of the leaf, which are successively developed 

 from above downwards, would have stomata of various degrees of perfection. He 

 noticed that in the lower part of the leaves, or that most recently developed, small 

 quadrangular cells, with a slit of about equal diameter either way, were placed between 

 the layers of the epidermis. These sometimes contained a granular substance, which, 

 higher up in the leaf, became a compact mass. At the same period a partition waa 

 formed in the middle of the cell, at first slightly, but subsequently more strongly 

 marked, and at length unfolded, so that the simple cell became divided, and a stomate was 

 formed. After this the surrounding cells enlarged, and the central slit increased at a still 

 greater rate. All this and the subsequent completion of the stomate may be observed by 

 any of our readers who may have a tolerable microscope, and will obtain, by practice 

 a certain delicacy in cutting minute structures. 



