THE GYMNOSPERMAE : CONIFERALES AND TAXALES 673 



single throughout the leaf. The pericycle is made up of a number of types 

 of cells. Firstly, there are parenchymatous cells rich in proteins, which are 

 called albuminous cells ; they abut upon the phloems of the vascular 

 bundles. Secondly, there are cells with bordered pits and no contents, which 

 are termed tracheidal cells ; they lie alongside the bundle xylems. Thirdly, 

 there is typical parenchyma in which these two types of cells are embedded. 

 These special tissues are called collectively the transfusion tissue. This 

 is characteristic of the leaves of Conifers and apparently assists in the diffusion 

 of fluids to and from the mesophyll. Probably it makes up for the poor 

 development of vascular bundles in the leaf. 



The apparent absence of any intercellular spaces in the mesophyll, as 

 seen in transverse section, is explained when a longitudinal section is examined 



Mesophyll 



Sieve tube 



Tracheid 



Endodermis __£ 



Fig. 672. — Pt72ns sylvestris. Longitudinal section of leaf 

 showing the lamellar structure of the mesophyll. 



(Fig. 672). Then we see that the mesophyll consists of transverse plates of 

 cells, one cell thick, and that the plates are separated, sandwich fashion, by 

 spaces which run right across the leaf and provide adequately for the circula- 

 tion of air to the mesophyll cells. 



Reproductive Organs 



The reproductive organs consist of spores which are produced in sporangia, 

 borne on specialized structures which are at least partly sporophyll in nature 

 and are aggregated together into the cones. Although these structures are 

 sporophvtic in origin and therefore cannot be regarded as being in any way 

 sexual in character, it is customary- to refer to them as male and female cones. 

 The male cones are borne in clusters at the base of the youngest shoots. They 

 appear in the earlv spring and produce microsporangia in which microspores 



