METABOLIC PROCESSES IN THE PLANT. 339 



half of the fruit is filled with endosperm in the midst of which is 

 the embryo. We farther see the five main ribs of each mericarp. 

 In the tissue of the wall of the fruit we perceive the vittae, which 

 ure intercellular spaces, filled with ethereal oil. 



To observe resin passages we may examine very delicate trans- 

 verse sections of the needles of Pinus sylvestris. The epidermis, 

 whose cells are very strongly thickened, is followed by a layer of 

 hypoderm cells. At the two angles of the leaf this layer is more 

 strongly developed. The resin passages (quite a number of them 

 are always present) lie in the green tissue. Each resin passage is 

 lined by a layer of thin-walled cells, the epithelium, which with- 

 out doubt yield the material from which the secretion is formed, 

 and furthermore, each resin passage is surrounded by a layer of 

 strongly thickened sclerenchyma fibres. We see also the green 

 tissue of the leaf, and the tissue, almost devoid of chlorophyll, in the 

 middle of the leaf, which is separated from the green tissue by an 

 endodermis. The nearly colourless ground tissue of the middle 

 of the leaf is seen to be made up of thick- walled and thin-walled 

 elements, and encloses two vascular bundles. The leaf of Pinus 

 Pinaster is similar in construction to that of P. sylvestris. Its 

 leaves are better for section-cutting than those of P. sylvestris, 

 <ind if they are available we give them the preference for this 

 reason. It is also very easy to make out the presence of resin 

 passages in the stems of many Umbellifera?. We examine, e.g., 

 under a low power, transverse sections from the flower stalk of 

 Fo3niculum officinale. The epidermis, the cortex, the bast and 

 wood of the vascular bundles, and the pith are clearly seen. The 

 resin passages are situated in front of the vascular bundles. They 

 lie in the cortex between the fibro-vascular strands and a tissue 

 which we at once recognise as collenchyma. 



1 See de Vries, Landwirthschl. Jahrbiicher, Bd. 10. 



135. Colouring Matters. 



Many plant structures contain pigments of very different kinds. 

 In the first place it must be emphasised that these pigments are 

 of no slight physiological interest, from the mere fact that many 

 of them indicate directly the reaction of the cells in which they 

 .are found. In the cell-sap of the hairs on the leaf-stalk of many 

 Begonias are dissolved red pigments, whence we conclude that the 



