MORPHOLOGY OF HIGHER PLANTS. 373 



isolateral leaves the palisade cells occur beneath the epidermal 

 layers of both leaf surfaces, as in senna. 3. In some leaves, as 

 in Eucalyptus, the entire parenchyma is made up of palisade 

 cells. 4. In a few leaves there is no differentiation of a palisade 

 layer, and these are sometimes referred to as centric leaves. The 

 palisade cells may contain not only chloroplastids but crystals of 

 calcium oxalate (Fig. 215), tannin-inclusions (Fig. 114), etc. 

 Distributed among the palisade cells may be the oil-secretion 

 reservoirs (Fig. 216, B). Furthermore, the palisade cells may 

 be of equal length or the stratification may be quite uneven and 

 irregular. In shape they may vary from long, narrow cells to 

 short, broad cells. In some special instances they are narrowed 

 at the lower end in the form of a blunt cone forming the so-called 

 " funnel cells," which are especially characteristic of plants in- 

 habiting moist localities. There is still another common form 

 known as arm-palisade parenchyma, in which the cells are branch- 

 ing and connected with each other by means of the branches. 

 Tissues of this type occur in the Equisetaceae, Filices, Coniferse, 

 Gramineae, and in a number of Dicotyledons, such as Aconitum, 

 Adonis, Anemone, Caltha, Clematis, Delphinium, Nigella, Paeonia, 

 and Trollius in the Ranunculaceae ; Sambucus and Viburnum in 

 the Caprifoliaceae; Lysimachia and Trientalis in the Primulaceae. 

 The spongy tissue or dorsal pneumatic tissue shows consider- 

 able variation in the arrangement and shape of the cells. In some 

 leaves the cells are arranged in strata or layers, while in others 

 they are more or less irregular. The cells may be spherical or 

 provided with a number of arms, the latter developing parallel 

 to the surface of the leaf or radiating in any direction, thus caus- 

 ing a variation in the nature and size of the intercellular spaces. 

 In some instances there are included in the mesophyll certain 

 mechanical cells, of which the simplest are like ordinary stone 

 cells. They may be more or less elongated or branched or even 

 quite fibrous, and are known as SPICULAR CELLS. The latter are 

 sometimes quite prominent when they traverse the leaf in a verti- 

 cal direction, giving rise to translucent spots. Spicular cells have 

 been found in the mesophyll of quite a number of families. They 

 are quite characteristic, although absolutely not constant, in the 

 r genuinp tea leaf (Thea sinensis). 



