414 Mr Thomas, On the assimilating tissues 



The stomata were confined to the adaxial side of the leaf, 

 which in the living state was probably also the lower side. Here 

 they occur in considerable numbers, one example showing a 

 distribution of about 440 per sq. mm. They were about '024 mm. 

 long and '018 mm. broad. In shape, the guard cells resembled 

 very closely those of modern Equisetums, and in one specimen 

 they show also the transverse striations of thickening material, 

 which are so characteristic of the recent plants. The guard cells 

 are usually level with the surface of the epidermis but may be 

 slightly sunk in it. The latter tissue forms a well-marked feature 

 in the leaves. 



Towards the centre of the leaf the pallisade cells abut directly 

 on the bundle sheath, and are often somewhat enlarged at the 

 point of contact. This enlargement is a feature sometimes ob- 

 served in modern plants, and Haberlandt has figured a similar 

 arrangement in the connection between the "Zuleitungsystem " 

 and the " Ableitungsystem " in Gycas circinalis*. There can be 

 little doubt that the bundle sheath, termed by Hickf "melasmatic 

 tissue," functioned as the patli of conduction for the products of 

 assimilation. It is a conspicuous feature in most specimens, 

 because its cells are characterised by the large mass of opaque 

 carbonaceous matter which they contain. No distinct tissue 

 can be recognised which could be called phloem, nor can any 

 sieve tubes be made out. Some leaves show thin-walled elongated 

 parenchymatous cells associated with the xylem, while in others 

 this feature is completely absent. In each leaf, however, the 

 amount of assimilation could not have been large, for the leaves 

 are very small; the products could easily be conducted the short 

 distance to the stem by diffusion through the elongated cells 

 of the bundle sheath. 



The cells of the thin-walled inner zone of the cortex of the 

 young stems often contain black contents like those of the bundle 

 sheath, and it may be suggested that these also were connected 

 with the formation of carbohydrates. 



It would appear then that in the Calamites the leaves show 

 a structure adapted for carbon assimilation, even though they 

 were very small. They were probably produced in large numbers 

 and were sufficient to provide for the nutrition of the plant. 



The leaves of other Coal Measure plants are now being ex- 

 amined. Some Lepidodendrons possess foliar members in which 

 we find a considerable proportion of mesophyll, which may be 

 either of a spongy type composed of rather stellate cells, or in 



* Haberlandt, " Verleichende Anatomie des assimilatorischen Gewebesystems 

 der Pflanzen," Pringsh. Jahrb. 1882, Taf. vi. fig. 15. 



t Hick, " On the structure of the leaves of Calamites," Mem, and Proc. Man- 

 chester Lit. and Phil. Soc. Vol. ix. 1895, p. 179. 



