268 



THE YOUNG NATURALIST. 



of wheat and grasses. The floral leaves 

 compose the flower, the anatomy of 

 which would require a separate article. 

 The seed-leaves are the first leaves of 

 the infant plantlet, and they are 

 folded up in the seed. These may be 

 either thick succulent and fleshy, like 

 those of peas and beans, where they 

 form the great bulk of the seed, and 

 contain a suflicient source of nutriment 

 for the young germinating plant. In 

 such cases as a rule they remain 

 underground, their substance becoming 

 changed into assimilable nutriment 

 which is absorbed by the young growing 

 plant. In other cases, as in mustard 

 and cress, turnips, cabbages and 

 sycamores, they rise above ground, and 

 growing green perform for a time the 

 function of nutritive leaves, which we 

 shall now consider. To the naked eye 

 an ordinary leaf is seen to have a 

 skeleton or framework of veins 

 (prosenchyma), with all the interstices 

 filled up with softer tissue (parenchy- 

 ma), and the whole upper and under 

 surfaces covered with a skin (epidermis), 

 which can be peeled off. Yiewed 

 under the microscope the epidermis 

 instead of being continuous is seen to 

 be pierced with innumerable little 

 apertures (stomata), exquisitely formed 

 with two independent guard cells, 

 which have the power of opening and 

 closing the pores under varying 

 conditions of light and moisture. 

 These stomata are very irregularly 



distributed, being generally most 

 abundant on the underside, in leaves 

 with polished upper surfaces like the 

 cherry laurel, as many as 90,000 may be 

 seen on a square inch of the lower 

 epidermis whilst there are none on the 

 upper. In the floating leaves of water 

 plants, as the water lily, this proportion 

 is exactly reversed. In submerged 

 leaves, like those of the pondweed 

 (Potamogeton) , they are entirely 

 absent, whilst on grasses there is an 

 equal number on both surfaces. A 

 microscopic cross section of a leaf 

 would disclose under the upper 

 epidermis or skin, a layer or two of 

 closely ranked rather elongated cells, 

 with their longest axis perpendicular to 

 the surface, in other words like rows 

 of eggs set on end ; this is called the 

 palisade parenchyma. The rest of the 

 tissue of the leaf is composed of loosely 

 and irregularly packed cells with 

 considerable cavities between them. 

 From each stoma there is a little canal 

 leading through the palisade parenchy- 

 ma, so as to allow a free admission of 

 air into the intercellular spaces ; indeed 

 the whole substance is permeated with 

 channels like a sponge allowing of a 

 free transmission of air to the interior of 

 the leaf, precisely as it is admitted into 

 the lungs of animals in the act of breath- 

 ing. Tunctionally the leaves are the 

 lungs of the plant, and in a modified de* 

 gree they perform work exactly anala- 

 gous to the respiration of animals. 



