i879-] THE GARDENER'S PRIMER. 331 



node on the under side, and so forming a layer which will soon emit 

 roots. It is thought that the presence of vascular tissue at the node is 

 the cause of the rooting faculty of the cutting or layer. A cutting 

 made of the internode of the stem only, without the node, will not 

 grow ; nor will any cutting, however made (with or without a node), 

 of a monocotyledonous plant grow. 



In complete union with the stem is the leaf, which in land-plants 

 is formed of parenchyma, traversed by veins of fibro-vascular tissue, 

 furnished at its upper surface with a covering or epidermis, then with 

 a layer of cells flattened by the pressure of the atmosphere, then a 

 layer of cells filled with chlorophyll granules, and then air-cells, and 

 again an epidermis filled with stomata or breathing pores forming the 

 under surface of the leaf. The leaves of aquatic plants are formed 

 differently — the distribution of veins in the leaves is called venation, 

 and for the most part they are reticulated in the dicotyledonous 

 plants, and parallel in monocotyledonous plants; but some families, 

 such as Smilaceoe, have not unfrequently reticulated leaves. They 

 are usually described according to their position, such as opposite, or 

 alternate ; or to their mode of insertion, as by a stalk, or as sessile, or 

 as sheathed, or whether simple or compound (the descriptions of the 

 forms and names given to leaves are too long for insertion here), or 

 whether they have any appendages or not ; and the edges of leaves, 

 called circumscription, have been found to possess capital character- 

 istics for their descriptions ; sometimes they are annual ; and the 

 trees bearing leaves subject to the process of annually dropping or 

 shedding their leaves are called deciduous trees. (Sometimes the 

 leaves continue longer than a year, and the trees are then called 

 evergreen or persistent — that is, not falling off. Some leaves — not, 

 however, strictly leaves — have on the centre midrib the flower, such 

 as Ruscus aculeatus. There is a popular notion that every leaf 

 originates in a cell of the stem, and that with increase of cells comes 

 increase of foliage ; and that each leaf has a corresponding root, and 

 that with increase of foliage there must be increase of roots. It is 

 difficult to trace the origin of these ideas. 



The leaf containing in its cells chlorophyll, exercises a great power 

 in the life of the plant. By the performance of some hidden function, 

 due to cell-life in the presence of sunlight, the plant obtains its car- 

 bon, and the process is said to be as follows : during the action of 

 sunlight upon the leaf of a plant, the green chlorophyll granules per- 

 form some function, not yet clearly shown what, by which carbon 

 dioxide (carbonic acid), absorbed by the leaf from the atmosphere, is 

 decomposed, and the carbon is retained, probably in conjunction with 

 hydrogen in the water of the cell, some oxygen is set free, and some 



