74 



LECTURE VI. 



chambers appear more extended in the longitudinal direction (especially those 

 marked {g) in the figure) in the form of very long narrow tubes. 



A careful comparison of the two figures — i. e. the transverse and longitudinal 

 sections — leads to the conclusion that the various linear networks of the longi- 

 tudinal and transverse sections are due to the presence of chambers closed on all 

 sides, the cavities of which are separated from one another by solid walls, just 

 like the rooms in a large building. The transverse section of a portion of a plant 



thus corresponds to the ground- 

 plan of a building; the longitu- 

 dinal secdon to the elevation of 

 the same. 



Apart from those Algs and 

 Fungi which we have already 

 learned to recognise as non-cellular 

 plants, we find in all plants and 

 parts of plants this chamber-like 

 structure; and we still designate 

 the single chambers by the same 

 term which their discoverer, Robert 

 Hooke (1667), employed for them, 

 because he was struck by the 

 similarity of the chambers with 

 the cell-structure of a honeycomb. 



Fig. 71. — Transverse section of the flower scape of Allium Schoaio- vy^ chall r>f rmir<;P tSPP fhnt tVlic 

 /mx«;«(X30). ^ epidermis ; M cells containing chlorophyll ;»- colourless ^^ ^ ''"''-'^ "^ COUFbC bCC UldL LUli, 



oHcTeSyrnr"'' "' p"" P^^^"'^"^""^ = ^ ^' ™='^"'^-- ^""'"« • ^'^ ""«: similarity is an extremely super- 

 ficial one, although in the cells 

 of plants, as in those of a honeycomb, living contents become formed, and formative 

 nutritive materials are stored up. 



If we regard the longitudinal or transverse section of a very young portion 

 of a plant, such as a growing point (Fig. 72) or the transverse section through 

 ordinary wood or cork, we remark that all the cells fit closely into one another on 

 all sides ; since between any two neighbouring cells there is always only one solid 

 wall, like a simple wall between two chambers of a house. This kind of 

 cellular structure originally exists in every cellular plant, and accords with the 

 origin of the cells during growth. But very often it happens that in the course of the 

 further development, the chambers separate partly or entirely from one another, the 

 simple primary partidon walls becoming more or less split ; and each single chamber 

 thus becomes bounded by a wall of its own, at certain places, or over its whole 

 circumference. This case is observed very generally in the tissue of green leaves, 

 especially in the lower half of the thin lamina. The cells, at first fitting closely on all 

 sides, have separated from one another during the growth of the leaf, and are now 

 connected only at isolated points. The case is similar also in the seed-coat of the 

 Gourd, of which Fig. 73 represents a transverse section. The inner layer of this 

 seed-coat consists of cells which have eventually become separated from one 

 another, since through growth they have assumed various irregular forms. In such 

 cases empty spaces exist between the now independent chambers or cells, which are 



