34 A TEXTBOOK OF THEORETICAL BOTANY 



GYMNOSPERMAE 



Cyras revoluta . . .24 Ephedra distachya . . 24 



Ginkgo biloba . . -24 Gnetum gnemon . . 24 



Pimis sylvestris . . .24 WeJwitschia mirabilis . . 50 ? 



Taxus baccata . . . 16-24 



ANGIOSPERMAE (Selected) 



Crepis capillaris . . .6 ConvaUaria niajalis . -38 



Ranunculus acris . . -14 Senecio vulgaris . . .40 



Bellis peremiis . . .18 Cerastium vulgatum . -144 



Quercus robur . . .24 Rumex hydrolapathum . . 200 



OUTLINE OF THE HISTORY OF CYTOLOGY 



Cytology is one of the most actively advancing branches of biological 

 study and we are far from having reached even an adequate knowledge of 

 the working of cell processes, so that our descriptions must be regarded as 

 tentative and our interpretations of them as more tentative still. Yet, when 

 we look back over the last century we see that cytology has advanced through 

 certain well-defined stages, each marked by the recognition of some basic 

 fact, which has proved itself a starting-point for further discovery. This is 

 the touchstone of validity in science, that a discovery or a theory should prove 

 fertile in new discoveries. Viewed in this light we may note the following 

 phases of advance : — 



1. Robert Brown in 1831 discovered the existence of nuclei in cells. 

 This may be said to have been the first discovery of importance in cytology 

 since cells were first observed by Hooke in the seventeenth century. 



2. In 1846 came the recognition of cytoplasm as a characteristic of living 

 cells by von Mohl. 



3. In 1848 Wilhelm Hofmeister first observed that the cell nucleus may 

 resolve itself into microscopic rod-like bodies. Although he attached no 

 interpretation to the discovery, we may regard this as the discovery of the 

 chromosomes. He also discovered the nuclear membrane and the nucleolus 

 and observed their disappearance at nuclear division. Lastly, he discovered 

 the cell plate which becomes the new cell wall. 



4. Flemming and Strasburger in 1882 (Fig. 17) observed the splitting 

 of the chromosomes during division, and van Beneden ascertained that the 

 split halves are accurately distributed to the two daughter nuclei. Thus 

 the process of mitosis was clearly defined. 



5. Van Beneden in 1883 and Strasburger in 1888 discovered the reduction 

 division which is associated with sexual reproduction, and showed that sexual 

 conjugation is essentially the union of two reduced nuclei to form one, which 

 has double the number of chromosomes. This established the existence of 

 the cytological phases of the life-cycle, and the importance of the nuclear 

 substance as the material of heredity. 



6. In 1883 van Beneden came to the conclusion that chromosomes had 

 a permanent individuality, but this belief did not find a proof till much later. 



