480 



PHYSIOLOGY. 



with flat ends, as in the medullary rays of Dicotyledons, or with conical 

 or oblique ends, the common form of wood-cells. Less frequent are the 

 forms of spores and pollen-grains, sometimes only temporary, sometimes 

 permanent, arising from the development of four cells by segmentation of 

 a spherical parent cell ; these sometimes appear of the form of quarters of 

 an orange, sometimes as tetrahedra, the curved surface forming the base of 

 the pyramid. In the tabular forms of the cell, the mutual pressure gene- 

 rally confirms an originally rectangular figure, the tabular cells of epi- 

 dermis and cortical structure being usually of quadrangular or polyangular 



Fig. 527. 



Fig. 528. 



Fig. 527. Young prothallium developed from the spore of a Fern (Pteris serrulata). 



Magn. 200 diam. 

 Fig. 528. Epidermis of the lower surface of the leaf of Helleborusfoetidus, with stomata 



(a). Magn. 200 diam. 



figure, flat above and below ; but in these we have sometimes complica- 

 tion from expansion, under pressure, principally in certain directions, 

 cells of the epidermis of many plants exhibiting side walls thrown into 

 sinuosities following a particular pattern (fig. 528). 



By far the great majority of cells in the higher plants originate 

 in forms analogous to those produced by pressure, since they mul- 

 tiply by division, and the septa dividing two newly formed cells 

 have ordinarily plane surfaces (fig. 527) : a spherical cell forms two 

 hemispherical cells, &c. ; a prismatic cell dividing perpendicularly, 

 two half-prisms, or, if horizontally, two superposed shorter prisms, 

 &c. As a general rule these cells have a tendency to assume the 

 spherical (or cylindrical) form in their earlier stages of growth, 

 while the whole mass of tissue is lax ; and if they are set free, as 



