116 NATURAL mSTOBY OF PLANTS. 



the starch p^ranules are irregularly rounded, and of very unequal 

 size. In most species of JFormia the pith becomes hollowed when at 

 a certain age, forming nearly parallel septa or leaving a cavity of 

 irregular form. Tlie pith, though much flattened, is not wanting 

 in the species with cladodia analogous to those of Xylojjhjlla, and 

 especially in those of the genus Fachynema ; here the fibro- vascular 

 woody bundles are naturally nearly parallel, diverging towards the 

 pulvinus so as to simulate the lateral ribs of a leaf. 



" The most remarkable feature in the structure of the BilleniacecB 

 is the frequent occurrence of fibres with areolate punctations,* 

 the areolae surrounding the perforations being found in every 

 degree of development, according to the age and species of the 

 specimen. Thus in a very young herbaceous branch of Billenia 

 speciosa, we only find common woody fibres, accompanied in every 

 bundle by vessels of all kinds, especially cylindrical vessels with 

 very thin walls, strengthened by pretty thick parallel rings at 

 long intervals, and also true or false tracheae, in which we often see 

 the spiral thread become single for a variable distance, although 

 there are most usually two distinct parallel cords. At this period the 

 cortical parenchyma is very rich in tubular cells of the herbaceous 

 layer, full of enormous chlorophyll granules, and the liber fibres 

 show minute punctations. The suber is formed of a fine compact 

 cellular tissue ; the epidermis is covered with simple hairs, swollen, 

 and, as it were, geniculate at the base. On a distinctly woody 

 branch of the thickness of the finger all the punctations of the cells 

 and fibres have assumed quite another character. The cells of the 

 medullary rays, full of starch inside, communicate with one another 

 extensively by cylindrical canals, punched, so to speak, in their very 

 thick walls. On the walls of the woody fibres the canals have the 

 form of a truncated cone with the small end outside ; two of these 

 truncated cones, at exactly the same level on two adjacent fibres 

 touch by these small ends ; and it is at the junction on a level with 

 the contracted part of the sort of hourglass thus formed that the 

 lenticular cavity is placed, easily seen on making a longitudinal 

 section. But when seen in front, it appears as it does in the Conifers, 

 as a very dark, circular or elliptical spot, surrounded by the con- 



' [Or so-called " glandular woody fibre."— Teaks.] 



