168 PHYSIOLOGY OF NUTRITION, 



and on microscopical examination of transverse sections we find 

 (see Fig. 58) that where lenticels occur the epidermis is burst open, 

 and that the lenticels are themselves filled with a powdery mass 

 of dark-brown coloured cells (packing cells). These packing cells 

 are, as is known, produced, as also are the cells of the phelloderra, 

 by the cambium of the lenticels. The packing cells, which as 

 rapidly as they undergo disorganisation from without are replaced 

 by the cambium, are so arranged that spaces are left between 

 them filled with air, and these communicate with the intercellular 

 spaces of the internal tissue of the twigs. To prove this fact 

 experimentally, we fasten a shoot of Sambucus nigra, Salix, or 

 Pavia rubra, covered with periderm arid lenticels, in the shorter 

 limb of a bent glass tube by means of a melted mixture of 2 

 parts wax and 1 part Colophonium. The cut surface at the top 

 of the branch is also well sealed with the mixture, and the pre- 

 paration is placed in a glass cylinder filled with water. If we 

 now pour mercury into the longer limb of the tube, bubbles of 

 gas quickly emerge from the lenticels. They gradually detach 

 themselves, only to be replaced by fresh ones. I have made 

 similar observations in winter on Sambucus material, from which 

 it is clear that the lenticels are not closed even at that time of 

 the year. In many plants, however, the newly formed packing 

 cells are more closely arranged in winter than in summer, so that 

 if we perform experiments, such as have just been described, in 

 December, and again at the beginning of June, using Ampelopsis 

 material, it is found that in June large quantities of air can be 

 forced through the lenticels far more easily than in December. 

 The lenticels in twigs covered with periderm play a role analogous 

 to that of the stomata on young structures. Like these, they are 

 of importance for the ventilation of the tissues. 1 We can also 

 determine this fact very simply by cementing shoots of the plants 

 at both ends with wax mixture, and then putting them in warm 

 water. The warmed air now escapes in bubbles from the lenticels. 



1 Literature : Stahl, Botan. Zeituny, 1873, and Klebahn, Jenaische Zeitschrift 

 fiir Medicin und Naturwissenschaft, Neue Folge, Bd. 10. 



67. Stomata and their Importance in the G-as Exchange. 



A very favourable object for the study of stomata is the leaf of 

 Iris florentina. On microscopic examination of delicate transverse 

 sections, it is seen (see Fig. 59) that the guard cells of the stoma- 



