APPENDIX. 475 



the protoplasm. The cells contain numerous chloroplasts and 

 starch-grains. The mesophyll is not differentiated into pali- 

 sade and spongy layers. In the mesophyll, immediately 

 under the hypodermis, are a number of resin-passages, each 

 with a thin-walled epithelial layer and an investing strengthen- 

 ing layer of sclerenchyma. 



In the middle of the leaf there is a conspicuous endo- 

 dermis surrounding a many-layered pericycle, in which two 

 vascular bundles are imbedded. The bundles are collateral, 

 the xylem facing towards the flat upper surface ; there is a 

 rudimentary cambium. 



The pericycle contains cells of two kinds : (a) parenchy- 

 matous cells containing protoplasm, proteid, and starch ; (6) 

 similar cells with bordered pits and no contents, resembling 

 tracheides and called tracheidal cells. This peculiar tissue is 

 called transfusion-tissue. It is characteristic of the leaves 

 of G-ymnosperms. It helps in the transference of nutritive 

 solutions, and thus makes up for the poor development of 

 vascular tissue. The tracheidal cells serve for the passage of 

 inorganic solutions from the vascular bundles to the meso- 

 phyll; the other cells for the diffusion of elaborated com- 

 pounds from the mesophyll to the phloem. In addition to the 

 transfusion-tissue, a number of fibres are developed in the 

 pericycle near the bundles. 



The general characters of the male and female flowers have 

 already been described (Art. 377). The male flowers differ 

 from those of most Angiosperms in that the axis (correspond- 

 ing to the receptacle) is elongated, there are two pollen- 

 sacs instead of four, and there is no perianth. The female 

 flower or cone, when young, shows a stout axis, which bears 

 scales of two kinds : (a) small scales arising directly from 

 the axis the bracts or cover scales ; (6) stouter scales deve- 

 loped one on the upper surface of each bract these are the 

 ovuliferous scales, each bearing two ovules on its upper side. 

 In comparing the female flower of the Pine to an Angiosperm 

 flower, the bract or cover scale is regarded as representing the 

 carpel, and the ovule-bearing scale as corresponding to the 

 placenta from which arise the ovules in the ovary of an 

 Angiosperm. 



When ready for pollination, the Pine ovule consists of a mass 

 of tissue called the nucellus, covered by a single integument 



