768 SUMMARY OF CUKEENT RESEARCHES RELATING TO 



dissecting Microscope. As a rule, it was then sufficiently transparent. 

 This method is especially to be recommended for examining the 

 younger stages of development ; older embryos require a longer treat- 

 ment with concentrated potash-solution, and a further addition of 

 alcohol or dilute glycerin. If the embryo has become too clear, dilute 

 hydrochloric acid may be added, and the preparation then coloured 

 by hfematoxylin or solution of eosin, with final addition of dilute 

 glycerin. Ammonia may also be used, alone or in conjunction with 

 potash-solution. 



CoUenchyma.* — E. Giltay states that, although in some cases 

 collenchyma may consist of a procambial or pseudo-procambial prosen- 

 chymatous tissue, in others its gradual development may be followed 

 out from a parenchymatous tissue. The original parenchymatous cell 

 may acquire, during its elongation, a more or less perfectly prosen- 

 chymatous form ; at first the collenchymatous thickenings make their 

 appearance only at the corners, subsequently, however, affecting the 

 whole cell-wall. When, as is more often the case, there is also ligni- 

 fication, a more or less typical sclerenchymatous fibre becomes 

 differentiated, independently of the more or less prosenchymatous 

 nature of the cell in the stage preceding lignification. The cell may 

 remain permanently at any of these stages of development. 



Very slight collenchymatous thickenings are but of small import- 

 ance to the cell. Although no sharp line of demarcation can be 

 drawn, in this category must be included the collenchymatous 

 thickenings which often occur in parenchymatous cells with large 

 intercellular spaces, generally clothing the intercellular space only ; 

 the parenchymatous cells with slight collenchymatous thickenings 

 ■which are found especially on those parts of the bark that are 

 adjacent to fissures ; and perhaps also in some cases the collenchy- 

 matous epidermis and the collenchymatous bast. 



What must be regarded as typical collenchyma almost always 

 occurs near the periphery of the organ, imparting to it its firmness and 

 elasticity. The collenchyma does not only, like sclerenchyma, carry 

 on as a whole a contest with the assimilating tissue ; its cells are also, 

 so to speak, at war with chlorophyll, which attempts to enter them, 

 causing them to revert to the condition of unthickened parenchymatous 

 cells bordering on intercellular spaces, in which cells the chlorophyll 

 best comes into play. A very sharp demarcation is frequently dis- 

 played between the thin-walled chlorophyllaceous cells of the cortical 

 parenchyma and the collenchymatous cells which they surround, and 

 which are completely devoid of chlorophyll. In the transitional forms 

 between these last and assimilating cells, we find slight thickenings 

 and intercellular spaces. These transitions are especially well seen 

 in plants with a typical sub-epidermal ring of collenchyma, in which 

 some cells which contain a small amount of chlorophyll indicate a 

 connection between the green bark and the stomata. From these 

 instances it may be seen that in general the development of chloro- 

 phyll and of collenchyma stand in inverse proportion to one another 



* Bot. Ztg., xxxix. (1881) pp. 1.53-9. 



