56 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Structure and Development. 

 Veeretative. 



Leaf-structure and Physical Factors.* — Edith S. Clements has 

 investigated the relation of leaf-structure to physical factors. Her 

 experiments were made with typical hydrophytes, mesophytes, and 

 xerophytes, and she draws the following conclusions. Tyi^ical mesophyll 

 consists of equal amounts of palisade and spongy tissue and moderate 

 air-spaces ; in hydrophyll the palisade-cells are absent, while in xerophyll 

 there are no palisade-cells, few air-spaces, and water-storage tissue often 

 occurs. Decreased light and increased water supply cause increase of leaf- 

 surface and decrease of thickness, the converse also being true. The 

 response of chlorophyll to light is the indirect cause of the variation in 

 the thickness of the leaf. The varying shapes of the cells is due to 

 the different arrangements of the chloroplasts in different lights. 

 Decreased light and increased water supply produce looseness of cell-forma- 

 tion, and vice versa. Also, since a covering of woolly hairs decreases light 

 and transpiration, this likewise results in loosely-formed tissue. Humidity 

 affects water-content and causes a change in the cuticle. Temperature 

 has an indirect effect, owing to its relation to humidity. Plants are 

 affected by the low humidities and short seasons of high regions, and 

 are dwarfed by extremes of any factor. Plasticity varies with different 

 species, the Compositfe showing the greatest stability. No law can as 

 yet be found as to the exact variation in the histology of the leaf with 

 definite differences in physical factors, for while in plastic species it may 

 be proportional, epidermal and morphological modifications must also be 

 considered. 



Life-history of Neottia.j- — (t. Peklo has investigated the life- 

 history of Neottia nidus-avis, and finds that the shoot-formations which 

 occur normally in the life-cycle of this orchid serve the purpose of 

 vegetative reproduction. These shoots are set free by disturbance of the 

 parts overlying the root-axis of the nest and its side-roots, and usually 

 by the loosening of the connection between these parts, which loosening, 

 in extreme cases, results in the destruction of the nidus, in which there 

 are still sufficient elements for a longer life. The mycorrhiza-fungus is 

 transmitted by the parent-plant to its descendant, but it is possible, 

 nevertheless, to induce it to thrive upon an artificial, nourishing sub- 

 stratum, and thus to cultivate it apart from its host. 



Reproductive. 



Embryology m Hieracium.| — 0. Rosenberg has studied the em- 

 bryology of Hieraciimi, and finds that in most seeds the tetrad-formation 

 is accompanied by a reduced number of chromosomes, and a few of the 

 embryo-sacs thus formed attain full development, being thus quite 

 normal. In general, however, the normal embryo-sac is crowded out 

 and an aposporous one formed. In exceptional cases, e.g. Taraxacum, 

 an apogamous embryo-sac is developed. In both of these latter cases 



* Trans. Amer. Micr. Soc, xxvi. (1905) pp. 19-94 (9 pis.). 



t Flora, xcvi. (1906) p. 260-75 (2 tigs.). 



t Bar. Deut ch. Bot. Gesell., xxiv. (1906) pp. 157-61 (1 pL). 



