220 HOW PLANTS GROW. 



Form. — The normal form is spherical, or nearly so ; this is 

 rarely preserved except in the cases of unicellular plants, pollen- 

 grains, and the cells of loose pulp, such as Peach or Strawberry. 

 By mutual pressure, cells usually become polygonal, as in elder 

 pith. 



The two conditions affecting cell-form are : — ist, Mutual pres- 

 stire ; 2nd, Variatio7i in vigour of growth — /.<?., nutrition. If 

 these be equal on all sides and at all points, the original form is 

 preserved ; with varying pressure, and more vigorous nutrition at 

 the ends than at the sides, are formed the ellipsoidal, oval, and 

 hour-glass shapes. This end-growth, carried yet further, gives us 

 the elongated, prismatic, and fusiform cells. Let nutrition ensue 

 more vigorously at the sides — i.e., favour breadth rather than 

 length, and we find the tabular and disc-shaped varieties. If 

 nutrition proceeds equally on all sides, but not at all poi?tts, the 

 beautiful stellate cell is produced, seen in all the JuncacecB, or 

 rush order, and in Euastrum, one of our water-weeds. 



Markifigs. — Varying nutrition is the great factor in producing 

 markings seen on cell-walls. The primary cell-wall is a thin, 

 transparent membrane. Increase in thickness is due to deposi- 

 tion, early in life, of several layers inside the primary one, called 

 secondary layers ; the thickness is fairly uniform everywhere, and 

 the transparency is retained ; at the same time fresh matter is 

 absorbed interstitially by intussusception. These layers may so 

 increase as to wholly fill the cavity. 



Now, uniform thickening obtains only in the early stages ; 

 afterwards, want of uniformity is seen, especially at corners and 

 edges, giving the cavity an appearance of being rounded off. 

 This may be on one side only, as in Mistletoe epidermis. It is 

 usually seen at certain spots, forming pitted cells — ie., cells with 

 pits — which, if the pit extend from wall to centre, become canals 

 (or pore-canals, as they are called), simple, as in Clematis pith, 

 or branched, as in the shell of Walnut. If the canals in two 

 adjacent cells meet, the membrane between them frequently 

 vanishes and the cells become perforated, the canals becoming 

 widened at their base, or point of junction, into bordered pits, the 

 marking so characteristic of the wood of the pine order. 



According to the development of these pits, or of the second- 



