1^5 



With the exception of one or two rarely occurring on the lower dorsal 

 surface, stomata are only to be found in depressions on the ventral concave sides 

 of the concrescence, and where they occur in longitudinal irregular rows along 

 the whole extent of the ventral face of the concrescence, as shown in Figure 

 78 — the oval bodies on the left of plate. A few do, however, sometimes occur on 

 the appressed lower part of the free portion of the leaf. Being thus placed in the 

 channels, they have the full advantage of the whole leaf substance as a protection 

 against solar rays, rain, or cold; and perhaps a secondary protective provision is 

 provided, as the edges of the individual leaves have the power of closing the 

 entrance to the cavity whenever adverse aerial conditions prevail, for the sections 

 examined seemed to support this theory, as the apertures are sometimes found 

 open as well as closed (vide Figures 65 to 75). This of course can only be 

 verified by assiduous field observations, but nevertheless we are at present under 

 the impression that this may be one of the physiological significances of the 

 decurrence in Conifer leaves, i.e., that the maximum amount of protection for 

 the transpiratory surface is obtained by the minimum amount of leaf movement. 



The specific name was given by Brown on account of the bloom of the 

 leaves, as stated above, but Francis Darwin, " Journ. Linn. Soc," Bot., vol. xxii, 

 1886, p. gg, states, " The position of the stomata in Conifers is very generally 

 indicated by the existence of a glaucous bloom," but this is not so in the case of 

 this species of Callitris, for the stomata-bearing surfaces are practically hidden, 

 and cover too small an area to characterise the tree when they are exposed. In 

 this contention of ours, i.e., accounting for the concrescence in Callitris and the 

 functions of the conical epidermal cells and probable movement of the ventral 

 surface, the following quotation, we think, rather strengthens our views. In the 

 case of Pimis halepensis " the leaves of this tree in warm sunny weather are fully 

 separated, but if the sky becoriie overcast they close partially ; the sirocco pro- 

 duces a similar but more marked effect, but in rain the leaves collapse, giving the 

 tree a most melancholy aspect" (Moggridge, "Journ. of Bot.," Feb. i, 1867). 



Oil Cavities. 



When present these bodies are found to be situated in the upper portion 

 of the leaf concrescence, and in the middle of the leaf substance of that part. 

 They are obliquely fusiform in shape (Figure 77), a cross section showing a 

 circle or an ellipse (Figures 70 to 72), whilst their limited length bars them from 

 being classified as canals — a term used in describing oil containing bodies in most 

 other Conifers. To be exact, they occur in the lower portion of the spongy 

 tissue, and are not regularly distributed; sometimes one, and even two, thin- 

 walled reservoirs will be found in each leaf, whilst often only one or two of the 

 sections may show one. The cavities are all lined with thin-walled secretory 

 cells, backed by a circle of thick-walled protective cells ; they may be classed 



