84 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Tlio change from a unicellular to a multicellular condition appears to 

 be due to the influence of this external sheath. In Alga; the cellulose 

 cell-wall is formed in the middle of this sheath. In unicellular Algas 

 the tendency to form colonies is due to the copious secretion of mucilage, 

 which is external to, and quite distinct from the sheath ; and the primary 

 function of which appears to bo to prevent desiccation. This, again, has 

 its analogue in the higher plants in the copious secretion of mucilage 

 from the stipules of Anomoclada among Hepatica), and from the mucilage- 

 cells of Blechnum and Osmnnda. Plants remain unicellular so long as 

 the tendency of tho protoplasm to resolve itself into a sphere, after cell- 

 division, predominates over external forces ; and the same occurs whero 

 cells are free from the pressure of the surrounding tissues, as in pollen- 

 grains. 



The cap-like structure which covers the growing point in Oscillaria 

 is simply the relatively thick undifferentiated portion of the sheath, which 

 contracts as it becomes cuticularized. 



The ring-like structure at the distal end of the cells of (Edogonium 

 is described in detail, and is regarded by the author as only a special 

 form of apical growth, combined with an unusual rigidity of the investing 

 sheath. 



Influence of Light on the Form and Structure of Leaves.* — A 



series of experiments on the influence of various degrees of illumination 

 on the size and internal structure of leaves has led M. L. Dufour to the 

 following conclusions : — 



The development of the plant increases in proportion to the degree 

 of illumination. It increases in size, it branches more copiously ; its 

 stem and branches exceed in diameter the corresponding parts of the same 

 plant exposed to a less degree of illumination ; its leaves attain the 

 largest dimensions both in surface and in thickness ; and the flowering 

 is earlier and more abundant. 



The same law applies also to the internal structure of the leaf. The 

 stomata are more abundant. The elements of the epidermis are more 

 fully developed in the sun ; the cells are larger, with thicker lateral 

 and outer walls ; the cuticle, in particular, is more strongly developed. 

 The walls of the epidermal cells are more sinuous in the sun than in the 

 shade. The palisade-parenchyma also displays a stronger development ; 

 its cells are longer in the transverse direction than when the plant grows 

 in the shade ; they contain more chlorophyll and more starch. The 

 same also is true of the conducting tissue ; the vessels are more numerous 

 and larger. The strengthening tissue presents the same characters as 

 those displayed in the sclerenchymatous and collenchymatous elements. 

 The secreting canals are larger, and contain larger quantities of eli- 

 minated substances, and the same is true of the deposition of calcium 

 oxalate. 



As a general law, M. Dufour comes to the conclusion that the state- 

 ment of some previous observers that there is an optimum degree of 

 illumination for the plant considerably below that derived from the 

 direct rays of the sun, is incorrect ; and that, other things being equal, 

 the plant, and every part of the plant, is more fully developed in pro- 

 portion as it is exposed to a more intense illumination. 



* Ann. Sci. Nat.— Bot., v. (1887) pp. 311-413 (6 pis.). Cf. this Journal, 1887, 

 p. 824. 



