ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 371 



will sometimes produce from two to four sieve-tube-cells by transverse 

 division, and production of sieve-plates on the transverse walls. 



The formation of sieve-plates commences with the development of 

 symmetrical callus-warts on both sides of the terminal surfaces of the 

 cell ; the portions of cell-wall between these retain permanently their 

 original chemical constitution, and form the future cellulose-sieve of 

 the sieve-plate. A little later the callus-warts coalesc3 into a uniform 

 mass covering the cellulose-sieve, in which perforations appear in 

 place of the previous warts, causing a direct communication between 

 the contents of adjoining sieve-tube-cells. 



The period of existence of the sieve-tube may be divided, in 

 relation to its physiological function, into three epochs. The first is 

 the active period, characterized by the open sieve-plates covered with 

 callus, the parietal layer of protoplasm in the tubes, and the formation 

 within them of mucilage and sometimes also of starch-grains. In the 

 second or transition period the tubes lose their contents, and the 

 sieve-plate is covered by a homogeneous mass of callus, which soon 

 begins to become absorbed. The third or passive period relates to 

 those sieve-tubes the plates of which are again opened, but consist 

 simply of a cellulose-sieve without any deposit of callus ; the contents 

 have either entirely disappeared, or are often reduced to a small 

 quantity of mucilage, and the sieve-tubes can then at most only serve 

 for the transport of watery fluids. The relative length of these 

 different periods varies greatly in different plants. 



The author finally examines the structure of the sieve-tubes of 

 Monocotyledons, especially Typha latifolia and Phragmites communis. 

 In the rhizomes of PJiragmites the young sieve-tubes are developed 

 out of the procambial cells, which first divide by tangential walls into 

 two cells of unequal size ; the outer and larger of these developes 

 immediately into the sieve-tube-cell, while the inner and smaller one 

 divides, by a number of transverse and radial divisions, into cambi- 

 form. The young sieve-tube is at first distinguished from the neigh- 

 bouring procambial and young cambiform cells only by its larger 

 dimensions, and by having lost its power of division. But soon the 

 lateral walls thicken, and dots appear in them, and at the same time 

 wart-like prominences on the terminal wall, which are at first small 

 and are composed of pure cellulose, but gradually increase in size and 

 assume a callose character. The subsequent processes resemble 

 those in Dicotyledons. 



A great difference is observable between the behaviour of the 

 sieve-tubes in Monocotyledons and Dicotyledons. In the latter, 

 after having once become passive, they are constantly replaced by tlie 

 activity of the cambium, and therefore endure only for a few months, 

 or at most a few years ; in the former, in consequence of the absence 

 of cambium, the activity of the tubes lasts much longer, in fact, as a 

 rule, as long as the organ itself in which they are found. 



The author concludes with the following general remarks. The 

 elements of the sieve-tubes are always and everywhere prismatic, and 

 are either horizontal or sharply truncate at the extremities. Their 

 walls are always composed of pure cellulose, and arc never strongly 



