PHANEROGAMIA : FOLIAR ORGANS. 279 



When milk- vessels are present, they for the most part follow the vas- 

 cular bundles, lying on their under side ; yet isolated milk-vessels are to 

 be found dispersed through the parenchyma. If we compare the de- 

 velopment of the vascular bundles of the leaves with those of the axis, 

 we shall find, as the natural connection of the leaf and axis indicates, that 

 the under surface of the leaf corresponds to the bark ; and, agreeably 

 with this, we find at times the external layer of bark extending out for 

 some distance into the leaf. 



We have little to say respecting the structure of the pouch, for inves- 

 tigation is yet wanting to us here. In Nepenthes, as in many other 

 plants, the walls of the pitcher contain a large number of fine spiral 

 fibrous cells. In Utricularia, the intercellular spaces in the walls of 

 the pouch are strikingly large, and would be open, both internally and 

 externally, were they not always closed by one or two cells, like a stopper, 

 which bear upon their inner side peculiar four-armed hairs, and upon 

 their outer one or two plano-convex cells. 



4. All foliar organs, soon after their origin, exhibit a delicate 

 epithelium, which, in plants vegetating under the earth or under 

 water, is converted in time into epiblema, and in those vegetating 

 above the surface is converted into epidermis. Some parts of 

 flowers are clothed with a peculiar sort of covering, holding an in- 

 termediate station between epithelium and epidermis. We shall 

 have subsequently to speak of this. To the epiblema stomata are 

 wanting. The epidermis is commonly provided with them. In flat 

 horizontal leaves they are very frequently wanting on the epider- 

 mis of the upper side, and they are usually only found where a thin 

 or spongiform cellular tissue is present beneath the epidermis ; in 

 floating leaves, on the contrary, the upper epidermis only has stomata, 

 and through the upper layer of condensed elongated parenchyma, 

 air-canals pass into the under and thinner layer of parenchyma; as 

 occurs also in leaves that are surrounded with dense, elongated cel- 

 lular tissue. All parts usually known as appendages to the epi- 

 dermis are also found occasionally on the leaves: even the cork 

 structure is sometimes found on the petioles of long-enduring 

 leaves, as, for example, in some species of Potkos and Ficus, as 

 well as on the leaves of Crassula, Bryophyllum, &c. 



The cells of the epidermis are usually filled with a clear watery 

 fluid, which on the under surface of the leaf is sometimes colored 

 (red). They more rarely present crystals, and yet more seldom 

 offer any peculiar matter, as resin, or the like. The form of the 

 epidermis-cells is determined by the form of the leaf; long, slender 

 leaves usually present their epidermis cells elongated in the same 

 direction. The lateral walls of the epidermis-cells are often curved 

 in the form of waves, but this peculiarity has been too little in- 

 vestigated to be explained at present. 



On the structure of the epidermis and stomata, enough has been said in 

 the first part. Respecting the occurrence of the particular appendages of 



parenchyma, formed of large cells ; and this is destined by its laceration to form the 

 air-canals, e. g. Aiundo Donax. 



T 4 



