1883.] development of the Leaves of Pinus Silvestris L. 359 



istic reticular structure described by Schmitz 1 . The cells are 

 readily distinguished from those of the more peripheral cortical 

 tissue by the complete absence of chlorophyll corpuscles in them so 

 that they appear colourless, as well as by the early disappearance of 

 most of their protoplasm, and the characters of their nuclei. At a 

 comparatively late stage a development of bordered pits occurs 

 both on their radial-longitudinal, as well as on their tangential 

 and transverse walls ; these pits may be seen both in surface view 

 and in transverse and longitudinal section, hence this tissue has 

 been called " areolar tissue." They appear, (except on the cells of 

 the sheathing-layer immediately adjacent to the palisade tissue 

 where they are absent,) at a late period, being first prominent 

 when the cell-walls separating the guard-cells of the stomata have 

 become thickened but have not yet split. The mode of their 

 development has been well described by Sachs {loc. cit. p. 23), and 

 was first accurately recognized by Hermann Schacht 2 . Towards 

 the base of the leaf this medullary tissue disappears. Below and 

 also to a certain extent above the fibrovascular bundle are a few 

 thick-walled sclerenchymatous cells which MacNab and other 

 systematists of the group, term "liber-cells" though they belong to 

 the ground-tissue. 



**The Fibrovascular bundle. 



This is single, but divides immediately on entering the leaf 

 into two well-marked branches which are collateral in arrangement, 

 very closely approximated and separated by a primary medullary 

 ray ; they have a very simple structure and end blindly. As regards 

 their histological elements, the xylem which lies towards the upper 

 surface consists only of a few spiral and annular vessels ; and the 

 phloem, which is directed towards the lower and outer surface, con- 

 sists of bast parenchyma and sieve-tubes, i.e. of soft bast only. 

 No bast-fibres are present, and tracheides with bordered pits are 

 completely absent from the xylem 3 , hence these are probably not 

 present in the primary wood of the stem formed from the procam- 

 bium. No resin passages, as Van Tieghem has already remarked 4 , 

 are present in the portions of the fibrovascular bundles lying in 

 the leaf though they are present in the parts of these same primary 

 bundles which lie in the stem. 



1 Sitzber. d. niederrhein. Qes. in Bonn, 1880. 



2 De maculis in plant a rum vans, &c, Bonn, 1860. 



3 In this last I differ from Bertrand, who {loc. cit.) states they are present. 



4 "Memoire sur les canaux Se"creteurs des Plantes," Ann. des Sci. Nat., Series v. 

 Vol. xvi. 1872. 



