466 BOTANY OF THE LIVING PLANT 



the spindle-fibres appear to be drawn apart, and converge towards the poles, 

 thus constituting the basis of two fresh and equivalent nuclei (v.). 



The further changes consist in the reconstitution of the two new nuclei, 

 through steps the reverse of those at the outset. Presently the nuclei are 

 again seen to be delimited by nuclear membranes (viii.) ; the chromosomes 

 gradually recover the diffuse form (vi. vii. viii.), nucleoli reappear, and the two 

 nuclei correspond in appearance to the parent (ix.). Meanwhile granules 

 appear in the equatorial plane (vii.), which coalesce and form the partition-wall 

 dividing the new cells. 



By this mechanism the chromatin of the parent nucleus is very exactly 

 divided into equal halves. The view is generally held that special functional 

 importance attaches to the chromatin, and that it is the chief carrier of the 

 heritable characters. If that be so, a mechanism is present in the division 

 of the nucleus of the somatic cell by which the nuclei multiply without materi- 

 ally changing the quality, but only the quantity of their chromatin-substance : 

 since an equal share of the substance of each chromosome passes to each of 

 the new nuclei. It is this that lies at the base of the fact that somatic budding 

 repeats very exactly the characters of the parent plant : for, as has been 

 seen in Chapter XII., the buds all originate vegetativel}^ that is by multiplica- 

 tion of cells by somatic division ; and each constituent nucleus will accord- 

 ingly bear the full heritable qualities of the parent. 



Tetrad Division. 



It is, however, different with those rapidly successive divisions,- — first into 

 two and then into four, — -which occur once in each completed life-cycle of 

 any sexual plant. Such tetrad- division habitually precedes the formation of 

 spores, which may accordingly be styled carpospores, to distinguish them from 

 other propagative bodies. Pollen-grains, spores of Ferns and Mosses, tetra- 

 spores of Red Seaweeds, or of Dictyota, ascospores, and probably also basidio- 

 spores are examples of such carpospores. All these initiate the haploid 

 or gametophyte phase of the life-story ; and this fact draws special attention 

 to the details of the tetrad-di\dsion which brings them into existence. 



Fig. 393 represents a series of twelve accurate drawings, made under camera 

 lucida, of pollen-mother- cells of Allium ursinum, the Common Garlic, all 

 to the same scale, viz. x looo. They illustrate successive stages in the tetrad- 

 division, by which the reduction of chromosomes to half the original number 

 is effected ; and the leading features of it are of general application. 



1. The general cytoplasm is of a dense and finely granular appearance. 

 The nuclear membrane is well defined. There is a delicate network of threads 

 of linin, on which the chromatin is more or less evenly distributed. The meshes 

 of the network are bathed in clear nuclear sap, and at one point there is a 

 nucleolus. 



2. The spore-mother-cell has enlarged, and at the corners its wall is separat- 

 ing from those of the neighbouring cells. The nuclear sap has greatly in- ' 

 creased, but there is no noteworthy increase in the chromatin-bearing linin, ! 

 which now Hes mainly on one side of the nucleus, as a kind of tangle, involved , 

 in which are two nucleoli. This stage of nuclear-division in the tetrad 

 is known as synapsis. 



:i 



