56 



BOTANY 



seeds also, e.g. Flax, Almond, Nuts of various kinds, the reserve-food 

 is largely oil, and in many spores, e.g. most Ferns and Mosses, oil is 

 very abundant. 



Crystals. Lime-crystals are of common occurrence in plant-cells, 

 much the greater number being calcium-oxalate, which appears in two 

 forms (Fig. 36), either as needle-shaped crystals or Rhaphides, very 

 common in many Monocotyledons, or tetragonal crystals of different 

 forms. These crystals are not soluble in acetic acid, which quickly 

 attacks calcium-carbonate, but they yield readily to hydrochloric 

 acid. Small crystals of calcium-sulphate occur in the vacuoles in 

 certain Desmids, and in old leaves of the Fox-grape (Vitis labruscd) 

 there have been detected crystals of calcium-tartrate. Calcium-car- 

 bonate rarely occurs except as an incrustation of the cell- wall. Curi- 

 ous accretions of this substance, Cystoliths, are found in the leaves 

 of some plants, notably the India-rubber tree (Ficus elastica). 



K 



FORMS OF CELLS 



The simplest plants are single cells, either naked, motile ones, or 

 stationary and provided with a definite cell-wall. Such isolated 



cells are mostly globular or oval in 

 form, which is also the case with 

 the eggs and spores of the higher 

 plants, which represent the simple, 

 primitive type of cell. Such a cell 

 by growth and repeated division 

 gives rise to a simple cell-aggregate 

 or tissue, such as composes the 

 young parts of the higher plants 

 (Fig. 37). These young tissues 

 have cells of nearly equal longitudi- 

 nal and transverse diameters, or 

 are isodiametric, and have thin 

 cellulose membranes. The undif- 

 ferentiated cells become gradually 

 transformed into the permanent 

 elements making up the character- 

 istic tissues of the higher plants. 

 The progress of these changes can 

 be readily traced in longitudinal 

 sections or series of transverse 

 ones, passing through the apex of 

 a growing shoot or root. 

 Parenchyma. The commonest form of tissue is parenchyma, in 

 which the cells are thin-walled, and but little altered from their orig- 



FIG. 37. Apex of a shoot of Naias 

 flexilis, showing the arrangement of 

 the meristematic tissues; X, the 

 initial cell for the plerome, or 

 central cylinder of the shoot ; K, a 

 lateral shoot ; L, Z 2 , young leaves. 



