236 



EXOGENS. 



Root. 



Fig. CLXV. 



Stem. 



and a stem with a hollow woody axis surrounding cellular tissue, the whole 

 being covered in by a cellular integument. But as the woody cords are 

 merely plunged into a cellular basis, the latter passes between them in a 

 radiating manner, connecting the centre with the circumference by straight 



passages, often imperceptible to 

 the naked eye, but always pre- 

 sent. The annexed diagram 

 illustrates this. 



Here we have the origin of 

 pith in the central cellular tissue 

 of the stem, of wood in the woody 

 axis, of bark in the cellular in- 

 tegument, and of medullary processes in the radiating passages of cellular 

 tissue connecting the centre with the circumference. 



The woody axis is not, however, quite homogeneous at this time. That 

 part which is near the centre contains vessels of different kinds, particularly 

 dotted vessels (bothrenchyma) ; the part next the circumference is usually 

 destitute of vessels, and consists of woody tissue exclusively : of these two 

 parts that with the vessels belongs to the wood, properly so called, and 

 serves as a mould on which future wood is added ; the other belongs to the 

 bark, separates under the form of liber, and in like manner serves as a 

 mould within which future liber is disposed. 



At the commencement of a second year's growth the liber separates spon- 

 taneously from the true wood ; a viscid substance called cambium is secreted 

 between them ; and the stem again lengthens, forming new leaves over its 

 surface. The ligneous cords in the leaves are prolonged into the stem, 

 passing down among the cambium, and adhering in part to the wood and in 

 part to the liber of the previous year, the former again having vessels inter- 

 mingled with them, the latter having none. The cellular tissue that con- 

 nected the wood and liber is softened by the cambium, and grows between 

 them horizontally while they grow perpendicularly, extending to make room 

 for them, and consequently interposed between the woody cords of which 

 they each consist, forming in fact a new set of medullary processes termi- 

 nating on the one hand in those of the first year's wood, and on the other 

 in those of the first year's liber. This addition of new matter takes place 

 equally in the stem and in the root, the latter extending and dividing at its 

 points, and receiving the ends of the woody cords as they diverge from the 

 main body. The following figure illustrates this, and shows, when com- 

 pared with the last, what difference there is in the appearance of the stem 

 of an Exogen one and several years old. 



And thus, year after year, the Exogen goes on, 

 forming zone upon zone of wood, which is perma- 

 nent, and zone within zone of bark which perishes 

 at the outside, but is renovated at the inside, as 

 the stem increases in diameter. 



If this account is compared with what has 

 already been stated concerning Endogens, it must 

 be obvious that the stem of these two great classes 

 is formed from the very beginning in an essentially 

 different manner. Endogens have no cylindrical 

 column of pith ; their woody arcs are never col- 

 lected into a cylinder, through the sides of which the cellular tissue passes 



Fig. CLXV.— Ideal section of the root and stem of an Exogen one year old. 

 Fig. CLXVI. — Real section of the stem of an Exogen six years old. 



